Rocks and Minerals

Rocks and Minerals asks you to do two things quickly and correctly. First, identify a specimen from what you can observe: its hardness, streak, luster, cleavage, density and reaction to acid. Second, explain what that specimen tells you about how it formed and what happened to the Earth around it. The first skill is a toolkit, a short sequence of tests you run the same way every time. The second is a model of the rock cycle: magma cools, sediment is buried and cemented, and rock that is heated and squeezed changes without melting. Strong teams keep both in view. A correct name with no explanation of formation loses the reasoning points, and a convincing story attached to the wrong name loses the identification points.

Identification is worth at least 30% and at most 50% of the points, so the rest of the test is classification, data and explanation. The scope below follows the 2027 rules manual for Divisions B and C and the National Rocks and Minerals List, which is the same for both divisions. Divisions B and C share most of the topic outline. Division C adds several items (solid solutions, phase diagrams, silicate structures, crystal systems, the evolution of igneous melts, metamorphic facies and thin sections), and anything limited to State and National tournaments is marked in the text.

Event format

If the event runs as stations, the supervisor sets and announces the time at each station. You cannot go back to a station, but you may revise your answers while you are at later stations. The only material that may leave the binder is the National List.

What the rules cover

The table summarizes the 2027 rules manual (Rocks and Minerals B and C, rules 3.h–3.i) in plain words, and the last column says where this guide covers each topic. Check the manual on soinc.org for the exact wording.

Several lists in the rules are closed ("limited to"): the other properties, the habits, the igneous formations, the sedimentary textures, the index minerals, the gem varieties, the solid-solution groups and the thin-section minerals. A test that sticks to the rules will not go beyond them, so learn every item on each closed list rather than extra examples. The list of depositional environments is open, so other environments can appear. Supervisors also write "illustrate a concept" questions about minerals that are not on the list; this guide covers the common ones (for example andalusite, sillimanite, chlorite and serpentine) in the reference tables.

How tournaments test this

This breakdown comes from the scio.ly practice bank: 6,767 Rocks and Minerals questions from past tournaments, of which about 6,500 fall inside the 2027 rules (the rest are state-symbol trivia, earthquakes, meteorites and similar). About 3,400 of them match questions in the archive (which holds 9,102 Rocks and Minerals questions); most of the rest come from tournament tests the archive does not have. Each question was assigned the one topic a student must know to answer it. The bank undercounts identification: most identification is done from physical specimens at stations, which never reach a written question bank, yet the rules make identification 30–50% of the points. Read the table as the split of the non-identification points, and plan your study time with identification first.

• Format. Most questions are short free response: name the mineral, give the formula, explain a process in one or two sentences. Multiple choice, true/false and select-all make up the rest. Practice writing the process and the evidence in one sentence.
• Identification stations show a specimen or photo with a number. Some ask only for the name; many add a follow-up about the same specimen (its formula, hardness, crystal system, use, formation or a look-alike). That is why the reference tables carry every property, not just the ID tests.
• Typical shapes. A specimen described by its properties ("which mineral is it?"); a set of test results ("which result rules out X?"); a Hjulström reading; Bowen's order questions; a sedimentary environment inferred from grain size and structures; a protolith, index mineral or grade for a metamorphic rock; a rock-cycle process asked in words; a half-life or specific-gravity calculation.
• Most common answers. Olivine, calcite, limestone, quartz, basalt, granite, gneiss, pyrite, halite and marble. Know each one cold, including what it is most often confused with.

Strategy

• Split by skill, but rotate. At a station event, both partners move together. For paper sections, one partner takes identification and the other takes classification and data. Swap halfway so both of you can check the other's reasoning.
• Use a fixed identification flow. For minerals, start with luster (metallic or not), then streak, hardness, cleavage, reaction to acid and heft. For rocks, start with whether you can see crystals, then glass or vesicles, then clasts, then foliation. The flowcharts are in the Condensed facts.
• Explain in one or two sentences. A strong answer names the process and the evidence: "The rock is vesicular and fine-grained, so it cooled rapidly at the surface with gas trapped in the melt." Partial credit follows reasoning.
• Budget time. Station times are fixed and cannot be extended. If a specimen stumps you, write the best observation-based answer, then move on. Identification is capped at half the points, so you do not need every name to score well.
• Practice on the list. Go through the National List once, naming each mineral's streak, hardness and cleavage and each rock's texture and origin from memory, using the Identification gallery. Then practice with archived tests from several different writers.

What to bring and how to build your binder

You may bring one binder of any size, one field guide, an annotated National List, a magnifier and a Class II calculator. Everything you will look up at a station has to be findable in seconds, so the order of the binder matters more than its thickness.

The 2027 National List

Identification is limited to specimens on the 2027 National Rocks and Minerals List, although other rocks and minerals can be used to illustrate a concept. Read the list early and annotate your copy: it is one of the three references you may bring, and it is the only thing you may pull out of your binder at a station. A few features of the list change how you study:

• Starred specimens (*) are identified only at State and National tournaments. The official list stars 27 minerals: magnesite, rhodochrosite, siderite, pyrolusite, rutile, zincite, turquoise, pyromorphite, vanadinite, bornite, stibnite, actinolite, tremolite, apophyllite, chrysocolla, dioptase, lepidolite, prehnite, rhodonite, spodumene, jasper, smoky quartz, tiger's eye, stilbite, willemite, wollastonite and zircon. It stars 9 rocks: syenite, tuff, tuff breccia, diatomite, siltstone, greywacke, amphibolite, talc schist (soapstone) and serpentinite. Every other specimen can appear at any level.
• Specimens with supplied information (¹). Footnote 1 on the list marks six specimens, all of them also starred: the minerals magnesite and bornite and the rocks syenite, tuff, diatomite and siltstone. For these the supervisor gives extra information (diagnostic properties, a chemical formula, grain size or composition) to make identification possible. Your job is to match that information to the name; the drill below shows how.
• Naming conventions. The pink or tan potassium feldspar is answered as "Orthoclase/Microcline" or "potassium feldspar"; the rules also note that white or pink/tan orthoclase/microcline occurs in granite. Tourmaline is accepted for the whole group. Celestite may also be called celestine. Goethite/limonite and agate/chalcedony are each one answer. Rock salt and halite, and rock gypsum and gypsum, do not have to be told apart.
• Classification quirks. Bauxite is listed as a sedimentary rock, not a mineral. Opal sits among the quartz varieties even though it is an amorphous mineraloid. Grossular garnet is the green variety. Gypsum appears as three varieties: alabaster (massive), satin spar (fibrous) and selenite (crystalline).

Per-specimen data (formula, crystal system, hardness, specific gravity, luster, streak, cleavage, tests, formation, uses and gem varieties) are in the Mineral reference tables. The three mineral chapters that follow teach the concepts those columns rely on, so that you can reason from a property to a name, and from a name to an explanation: Mineral identification (the properties and tests), Mineral chemistry and structure (formulas, polymorphs, silicates, solid solutions, crystal systems) and How minerals form, ores, uses and gems.

Supplied-information specimens (¹)

A ¹ specimen usually arrives with a data card, because its look-alikes cannot be separated by eye. Learn which number on the card decides it.

Mineral identification

The 2027 rules open the mineral topics with a definition, and every physical property in this chapter follows from it. A mineral's composition (which atoms) and structure (how they are arranged and bonded) fix its hardness, cleavage, density, luster and many colors. If you know why a property arises, you can answer the "explain" questions that make up the largest share of archived tests, and you can rule specimens in or out instead of memorizing them one by one.

What counts as a mineral

Each word in the definition excludes something, and tests ask you to say which word a substance fails:

Mercury is the classic edge case: a native element that is liquid at room temperature yet is accepted as a mineral by the International Mineralogical Association for historical reasons. Answer "solid" for the definition anyway.

Species, variety and group. A mineral species is defined by its composition and structure (quartz). A variety is a named form of one species that differs in color, habit or trace elements but not enough in composition to be a new species (amethyst, citrine, rose quartz and rock crystal are all quartz; ruby and sapphire are both corundum). A group is a set of species with the same structure and related chemistry (the garnet group: almandine, grossular; the feldspar group; the amphibole group). A series is a group whose members grade continuously into one another (the plagioclase series from albite to anorthite).

Biogenic minerals. Organisms build mineral matter: calcite and aragonite in shells and corals, apatite in bones and teeth, opal in diatoms and sponges, magnetite in magnetotactic bacteria, and calcium oxalates in fungi and lichens, whose acids also lock toxic metals such as lead and uranium into insoluble minerals. The International Mineralogical Association excludes purely biological products such as a pearl or a living shell from mineral status (identical compounds formed geologically are minerals), so a test that asks "is a pearl a mineral?" wants "no, it is organic."

Fluoride and teeth. Tooth enamel is hydroxylapatite, Ca₅(PO₄)₃(OH), and F⁻ (same charge, almost the same size as OH⁻) substitutes for OH⁻ to make fluorapatite, whose more stable lattice dissolves less readily in the acids made by mouth bacteria. That is why fluoride is added to toothpaste and drinking water.

The test sequence

Run the same tests in the same order on every specimen. Consistency matters more than speed, and it makes your binder notes easier to read later. You may bring only a magnifier, so at a tournament you mostly observe luster, color, cleavage, habit and heft yourself; streak, hardness, magnetism and acid results are often given on a card or in the question. Practice the full sequence at home so you know what each result means when it is handed to you.

The rules say hydrochloric acid will not be used or provided and that a taste test is not allowed. Acid reactions therefore appear as written descriptions. Learn what the reactions mean, not how to perform them.

Hardness and the Mohs scale

A common mnemonic is "Tall Giraffes Can Fight And Often Quarrel Till Completely Defeated." The absolute values are the classic sclerometer figures (relative to talc and gypsum at 1 and 2).

Why minerals differ in hardness. Hardness tracks bond strength and how the bonds are arranged. Diamond's carbon atoms are each covalently bonded to four neighbors in a three-dimensional network, so a scratch must break many strong bonds. Graphite is the same element, but its sheets are held together only by weak van der Waals forces, so the sheets slide (hardness 1 to 2). Talc and the micas are soft for the same reason: strong sheets, weak bonds between them. Halite (2 to 2.5) is held by ionic bonds between singly charged ions; quartz (7) by a framework of strong Si–O bonds. Higher ionic charge, smaller ions and closer packing all raise hardness, which is why oxides such as corundum (Al³⁺ and O²⁻ in a dense structure) are very hard. Water in a structure lowers hardness: opal (5.5 to 6.5) is softer than quartz (7) partly because of its water and its amorphous, imperfect bonding.

Hardness can depend on direction. Because bonds are arranged differently along different crystal directions, a knife can scratch some minerals one way but not the other. Kyanite's two hardnesses (values in the kyanite row of the Mineral reference tables) are the classic identification clue; calcite is also slightly softer on its basal surface than on its cleavage faces.

Streak and luster

Streak is the color of a mineral's powder, made by rubbing it on unglazed porcelain. Powdering removes the effect of crystal size, surface coatings, tarnish and much of the scattering that makes a big crystal look dark, so the streak of a given mineral is far more consistent than its bulk color. Most nonmetallic minerals leave a white or colorless streak whatever their bulk color: amethyst, rose quartz and smoky quartz all streak white, as do calcite, fluorite, gypsum, feldspars, and the colored beryls. Streak is most useful for metallic and dark minerals and for a short list of nonmetallic minerals with colored powders.

Luster is the way a fresh surface reflects light. It depends on two things: whether the mineral absorbs and re-emits light at its surface (metals, with their free electrons, do, giving a mirror-like shine and opacity), and, for nonmetallic minerals, how strongly the mineral bends light (its refractive index) and the texture of the surface. A higher refractive index gives a brighter, more brilliant luster.

Why minerals have color

A mineral looks colored when it absorbs some wavelengths of visible light and transmits or reflects the rest. A handful of mechanisms account for nearly all mineral colors:

Diaphaneity and optical phenomena

Diaphaneity is how much light passes through a mineral. Transparent minerals let you read print through them (rock crystal, selenite, Iceland spar calcite). Translucent minerals pass light but blur the image (most chalcedony and agate, milky quartz, many feldspars). Opaque minerals pass no light even at thin edges (all metallic minerals, magnetite, pyrite). Diaphaneity can change with thickness, so hold a thin edge to the light before you call a dark mineral opaque. It is different from luster: diaphaneity is about light through a mineral, luster about light off its surface.

Several named optical effects appear on tests, often as "name the property":

Cubic (isometric) minerals such as garnet, fluorite, halite and diamond, and amorphous materials such as opal and glass, are isotropic: light travels at the same speed in every direction, so they show neither double refraction nor pleochroism. Every other crystal system is anisotropic. In general, anisotropic means a property varies with direction: optical behavior here, but also hardness (kyanite) and cleavage. This is why garnet in a thin section stays dark between crossed polarizers (see Thin sections for how this is used in Division C).

Cleavage, fracture and parting

Cleavage is the clearest example of structure controlling a property. Where a crystal has a plane across which bonds are few or weak, it splits there, and every parallel plane is equally weak, so it splits the same way again and again. Count the directions, not the faces: one direction of cleavage produces two parallel faces.

Cleavage quality describes how easily and how cleanly a mineral splits: perfect (smooth, mirror-like planes, hard to break any other way: mica, calcite, halite, galena, fluorite), good (clear planes but some fracture: feldspars, pyroxenes, amphiboles), fair or distinct and poor or indistinct (planes visible only in places: olivine, beryl's basal cleavage). This guide describes cleavage in words ("perfect basal", "two prismatic at 56° and 124°"). Some references add crystallographic plane labels in braces, such as {001} for basal; that notation is enrichment, not in the 2027 rules.

Fracture describes breaks that do not follow planes:

Conchoidal fracture is why early people prized flint, chert, jasper and obsidian: a blow detaches a curved flake with a razor-sharp edge, so the stone can be shaped into blades and points.

Parting produces flat breaks like cleavage, but along planes weakened by twinning, by thin exsolution layers or by strain, so only some specimens show it and it is not continuous through the crystal. Corundum (no true cleavage) commonly parts along basal and rhombohedral planes, and magnetite can part along octahedral planes.

Tenacity

Tenacity is how a mineral responds to stress such as hammering, cutting, bending or pulling. It is controlled by bond type: ionic and covalent bonds hold atoms in fixed positions, so the crystal snaps (brittle), while metallic bonds let layers of atoms slide without the crystal coming apart (malleable, ductile, sectile).

Density and specific gravity

Benchmarks to memorize. Knowing a handful of values lets you place almost anything:

Why the native metals are so dense. Gold, silver and copper (with lead, the gold group of native metals) combine both causes. Their atoms are heavy (atomic masses: Au 197, Ag 108, Cu 64), and metallic bonding lets them pack as tightly as identical spheres can: cubic closest packing, also called a face-centered cubic lattice, with atoms at the corners and face centers of each cube filling about 74% of the space. Heavy atoms in the tightest possible packing give the highest SGs on the list (gold 19.3, silver 10.5, copper 8.9). The shared structure is also why all three are isometric, malleable and ductile: close-packed planes of atoms glide over one another without breaking the metallic bonds.

Two habits of reasoning follow from the table. Within a solid-solution series, SG rises with the heavier element: fayalite (Fe) is denser than forsterite (Mg), and barite (Ba) is denser than celestite (Sr). And among polymorphs, the high-pressure form is the denser one: diamond 3.52 vs graphite 2.2, aragonite 2.95 vs calcite 2.71, kyanite 3.6 vs andalusite 3.15.

Measuring SG. Tests ask you to describe or compute four standard methods:

Other approaches include heavy liquids (a grain floats or sinks in liquids of known density) and a torsion balance such as the Berman balance, which works like a Jolly balance for small fragments.

Other properties: the closed list of eight

The rules limit "other properties" to eight. Know each one's meaning, cause, standard example, and how a question will describe it.

Luminescence family. Tests often ask you to distinguish related terms:

How a quartz watch keeps time. A quartz watch uses piezoelectricity both ways: an alternating voltage makes a tiny tuning-fork crystal flex, and the flexing crystal feeds a voltage back, locking the circuit to the crystal's resonant frequency (32,768 Hz, divided down to one pulse per second). Radios, computers and phones use the same oscillators, now cut from synthetic (hydrothermally grown) quartz because natural crystals are rarely free of twins and inclusions.

Heat tests: fusibility and flame color. Two older blowpipe tests sit outside the rules' list of eight but still appear on tests.

• Fusibility is how easily a mineral melts, judged by holding a thin splinter in a flame. It tracks bond strength, as hardness does: weakly bonded minerals melt at low temperature, while minerals built from strong, fully linked Si–O frameworks resist melting. Franz von Kobell's scale of fusibility (19th century) ranks seven reference minerals from easiest to hardest to melt: 1 stibnite (melts in a candle or match flame, about 525 °C), 2 natrolite, 3 almandine garnet, 4 actinolite, 5 orthoclase, 6 bronzite (an iron-bearing enstatite pyroxene), 7 quartz (infusible in a blowpipe flame). Like Mohs, it is a ranking, not a temperature scale.
• A flame test identifies a metal by the color it gives a flame when a powdered grain (often moistened with HCl) is heated, because heated electrons of each element drop back to lower energy levels and emit light of characteristic wavelengths.

Mineral habit and crystal form

Crystal structure, crystal form and habit are three different things. Structure is the internal, repeating arrangement of atoms (the lattice), and it fixes the mineral's symmetry. A crystal form is a set of equivalent faces related by that symmetry (cube, octahedron, prism); one crystal can combine several forms, as in a cubo-octahedron. Habit is the overall shape a crystal or aggregate actually takes as it grows, which depends on which forms dominate and on the growth conditions (prismatic, bladed, botryoidal). Fluorite has one structure, can grow cube or octahedron forms, and usually has a cubic habit.

A crystal grows euhedral when it has room and time: in an open cavity, in a melt early in crystallization, or as a porphyroblast (a large crystal that grows in the solid state inside a finer-grained metamorphic rock) (garnet and staurolite are classic euhedral porphyroblasts). Late-crystallizing minerals fill whatever space remains and are anhedral, which is why quartz in granite is usually anhedral while early feldspar is subhedral. Fast growth tends to give needles, skeletal or hopper crystals and branching dendrites; slow growth gives compact, well-faced crystals.

The tables also use descriptive words that are not on the rules' list of 21 (fibrous, reniform, mammillary, columnar, granular, foliated, lamellar, earthy); each is defined in the Mineral terms glossary.

Striations are fine parallel lines on a crystal face or cleavage. Growth striations record alternating growth of two crystal forms: pyrite cubes carry striations that run in different directions on adjacent faces, quartz prisms are striated across their length, and tourmaline prisms are striated along their length. Twin striations on a cleavage surface (plagioclase) come from polysynthetic twinning and are the best way to tell plagioclase from potassium feldspar in hand specimen.

Twinning

Twinning is the symmetrical intergrowth of two or more crystals of the same mineral that share some lattice points. The parts are related by a symmetry operation that is not a symmetry of the single crystal: a reflection across a twin plane or a rotation about a twin axis. (Twin laws can also be written in crystallographic plane and direction notation; like Miller indices for cleavage, that is enrichment.) Re-entrant angles (notches pointing into the crystal) are a quick visual clue, because a single crystal's faces never meet in an inward-pointing notch.

Twins form in three ways: growth twins (an error early in growth is copied as the crystal grows, as in staurolite and swallowtail gypsum), transformation twins (a high-temperature structure converts to a lower-symmetry one on cooling, producing the tartan twinning of microcline and Dauphiné twins in quartz cooled through 573 °C), and deformation or glide twins (stress shears the lattice, producing the twin lamellae in calcite of deformed marble and in plagioclase).

Mineral chemistry and structure

The previous chapter covered what you can observe; this one explains it from the inside. Families, formulas and polymorphs are tested at every level. Silicate structures, solid solutions and phase diagrams, and crystal systems are Division C topics, as their headings say.

Chemistry, groups and polymorphs

Atoms bond by reaching a stable, filled outer electron shell. In an ionic bond one atom gives electrons to another (Na⁺ and Cl⁻ in halite), and the oppositely charged ions attract. In a covalent bond neighboring atoms share electrons (C–C in diamond); the Si–O bond in silicates is about half ionic and half covalent. Metals hold their atoms in a sea of shared electrons (metallic bonding), which is why native metals conduct, are malleable and look metallic.

Mineral families. The family is set by the anion, because the anion group controls bonding and therefore most properties.

Reading a chemical formula

Formula questions test five skills: naming the family, counting atoms, assigning oxidation states, reading substitution notation, and computing a percentage by mass.

• Parentheses and subscripts. In Cu₃(CO₃)₂(OH)₂ (azurite) the subscript after a parenthesis multiplies everything inside: 3 Cu, 2 C, 6 + 2 = 8 O, 2 H.
• Substitution notation. (Mg,Fe)₂SiO₄ means the two cation sites hold Mg²⁺ and Fe²⁺ in any proportion; the element written first is usually the more abundant. (Ca,Na)(Al,Si)₄O₈ for plagioclase means Ca and Na share one site and Al and Si share four.
• Hydrated minerals. The "·nH₂O" in gypsum (CaSO₄·2H₂O) means water molecules are built into the structure (water of crystallization). Heating drives it off: gypsum becomes plaster of Paris (CaSO₄·½H₂O) and then anhydrite (CaSO₄). Opal's SiO₂·nH₂O has variable water (usually 6 to 10%), which helps make it a mineraloid. Hydroxyl groups (OH)⁻, as in micas, amphiboles, talc, kaolinite and serpentine, are not "water" in the formula but release water when heated; they also mark minerals that need water to form.
• Oxidation states. Find the unknown charge by balancing. Fe₂O₃: 3 × (−2) = −6, so each Fe is +3. Fe₃O₄: 4 × (−2) = −8 over three Fe, which is one Fe²⁺ and two Fe³⁺ (8 = 2 + 3 + 3). Pyrite FeS₂ contains the disulfide ion S₂²⁻, so Fe is +2. Goethite FeO(OH): −2 −1 = −3, so Fe³⁺. In swamps and other oxygen-poor water, iron travels as soluble Fe²⁺ and precipitates as Fe³⁺ oxides when it meets oxygen.

How chemistry changes properties. Composition controls density (heavy cations raise SG), color (transition metals), hardness (bond strength), solubility (halides and sulfates dissolve more readily than silicates), reaction to acid (carbonates), luster (metallic bonding in native metals and sulfides) and magnetism (iron). Trace elements, present at a fraction of a percent, mostly change color and fluorescence: corundum is colorless when pure, red with chromium (ruby) and blue with iron and titanium (sapphire). The chemical formula tells you what the element list should be, and the color tells you which trace element to suspect.

Polymorphs, isostructural minerals and pseudomorphs

Polymorphs are minerals with the same chemical composition but different crystal structures. Which polymorph forms depends on the pressure and temperature during crystallization: higher pressure favors denser packing. Because structure controls properties, polymorphs differ in hardness, density, cleavage and crystal system even though the formula is identical. (Polymorphs of a single element, such as diamond and graphite, are also called allotropes.)

Isostructural minerals (historically "isomorphous") have the same structure but different compositions, the opposite of polymorphs. Because the structure is shared, they show the same cleavage and crystal habit and often form solid-solution series:

A pseudomorph ("false form") is a mineral that has taken the external shape of another mineral, so its outward form does not match its own structure. Types:

• Replacement (substitution): the original mineral is dissolved and another precipitates in its place, molecule by molecule. Goethite/limonite after pyrite (rusty cubes), quartz or opal after wood (petrified wood), calcite after aragonite.
• Alteration: the original is only partly changed chemically. Malachite after azurite: azurite takes up water and loses CO₂ in the weathering zone, so malachite pseudomorphs after azurite are common and pure azurite is the less abundant of the pair.
• Encrustation: a second mineral coats the first, which later dissolves, leaving a hollow cast (quartz over fluorite cubes).
• Paramorph: a polymorph change keeps the old shape: α-quartz in the bipyramid form of β-quartz, calcite in the shape of aragonite.

Silicate structures (Division C)

Aluminum in the framework. Al³⁺ is close enough in size to Si⁴⁺ to sit in a tetrahedron, but it brings one less positive charge, so each substitution must be balanced by an extra cation. Replace one Si in four in quartz's framework with Al and add Na⁺ in the cavities and you have albite, NaAlSi₃O₈; replace two Si in four and add Ca²⁺ and you have anorthite, CaAl₂Si₂O₈. The same trick makes micas (KAl₂(AlSi₃O₁₀)(OH)₂) and the feldspathoid sodalite (feldspathoids are feldspar-like framework silicates with less silica, which form instead of feldspar in silica-poor magmas and never with primary quartz).

Cations between the tetrahedra. In the less polymerized structures, cations do the linking: Mg²⁺ and Fe²⁺ in olivine and pyroxene, Ca²⁺, Na⁺ and Fe in amphiboles, K⁺ between mica sheets. That is why the early Bowen minerals are Fe–Mg-rich (ferromagnesian or mafic: olivine, pyroxenes such as augite, amphiboles such as hornblende, biotite) and dark, and the later, highly polymerized ones (K-feldspar, muscovite, quartz) are Si-, Al-, K- and Na-rich (felsic) and light-colored.

Sheet silicates in more detail. Sheet silicates stack tetrahedral (T) sheets with octahedral (O) sheets of Mg, Fe or Al bonded to OH. In a dioctahedral sheet two of every three octahedral sites hold trivalent Al³⁺ (muscovite, kaolinite); in a trioctahedral sheet all three hold divalent Mg²⁺ or Fe²⁺ (biotite, talc, serpentine). Kaolinite and serpentine are 1 : 1 (T–O) layers; talc, the micas and pyrophyllite are 2 : 1 (T–O–T) layers; chlorite is 2 : 1 with an extra hydroxide sheet between layers. What holds layers together decides the properties: in talc, nothing but weak van der Waals forces (H 1, soapy), in micas a layer of K⁺ ions (stronger, elastic sheets, H 2.5), and in kaolinite hydrogen bonds (clay that is plastic when wet). The flat, reflective cleavage flakes of muscovite are why sheet silicates go into glitter and cosmetics.

Asbestos. "Asbestos" is a commercial term for minerals that grow as long, thin, flexible fibers (an asbestiform habit). Six are regulated: chrysotile (fibrous serpentine, a sheet silicate whose layers roll into tubes) and five amphiboles, including tremolite and actinolite (both on the list) plus crocidolite (blue asbestos, fibrous riebeckite), amosite and anthophyllite. The fibers resist heat and were used in insulation, brake linings, fireproofing and cement pipe. Inhaled fibers lodge in the lungs and cause asbestosis, lung cancer and mesothelioma; the straight, needle-like amphibole fibers persist in lung tissue longer and are generally considered more hazardous than the curly chrysotile fibers. The same minerals in massive or prismatic habit are not asbestos.

Solid solutions, ternary diagrams and phase diagrams (Division C)

Types of substitution.

The 2027 Division C solid-solution groups:

Other series you may meet in specimen questions: garnets (almandine Fe with pyrope Mg; grossular Ca with andradite Fe³⁺), biotite (annite Fe to phlogopite Mg), sphalerite (Zn with up to about a quarter Fe, which darkens it), and tourmaline (schorl, dravite, elbaite).

Naming plagioclase. Plagioclase is named by its percentage of anorthite (An), the Ca end member. A composition written An₆₅ or Ab₃₅An₆₅ is 65% anorthite and 35% albite.

A plagioclase An₆₅ is labradorite. The most calcic plagioclase crystallizes first from a mafic melt, and the albite end is the last; this is the continuous branch of Bowen's series.

The feldspar ternary diagram. A ternary diagram is a triangle with one end member at each corner: potassium feldspar (Or), albite (Ab) and anorthite (An). A point's distance from each side shows how much of the opposite corner's end member it contains: at a corner it is 100% that end member, and on the opposite edge 0%. The three percentages always sum to 100.

A miscibility gap (solvus) is a range of compositions where no single homogeneous crystal is stable: a mixture of that bulk composition is lower in energy as two separate phases. K⁺ and Ca²⁺ differ too much in size and charge to share a site, so the middle of the triangle is a gap. Labradorite sits at An₅₀–₇₀ on the bottom edge; anorthite, at the far end of that edge, is compositionally farthest from orthoclase, since it differs in both cation (Ca for K) and Al : Si ratio. A magma with K, Na, Ca, Al and Si will crystallize feldspar, and its position in the triangle predicts whether that is one plagioclase, one alkali feldspar, or both (as in granite, which has K-feldspar and Na-rich plagioclase side by side).

Binary phase diagrams. A phase diagram shows which phases (liquid, one or more solids, or a mixture) are stable at each temperature and composition. The manual limits you to two-component (binary) systems.

Read any binary diagram in four steps:

1. Locate the point for the bulk composition (horizontal axis) and temperature (vertical axis).
2. Above the liquidus everything is melt. Below the solidus everything is solid. Between them, crystals and melt coexist.
3. In a two-phase field, draw a horizontal tie line through the point. Its ends on the liquidus and solidus give the compositions of the melt and the crystals at that temperature.
4. Use the lever rule to find how much of each phase is present.

Equilibrium versus fractional crystallization. If crystals keep reacting with the melt (slow cooling, equilibrium), each crystal ends with the bulk composition (An₅₀ above) once the system reaches the solidus. If crystals are isolated from the melt, by settling or by being armored with new growth, the melt keeps evolving toward albite. The second case produces zoned plagioclase with calcic cores and sodic rims, and it drives the melt toward felsic compositions (see Evolution of igneous melts).

Eutectic systems. When two minerals do not form a solid solution, the liquidus has a V shape. Adding a second component lowers the melting temperature of each pure mineral (the same reason salt lowers the melting point of ice, and why fluxes and water lower the melting point of rock). The bottom of the V is the eutectic: the lowest temperature at which any melt in the system can exist, where the melt crystallizes both minerals at once at a fixed temperature and composition. The classic igneous example is diopside–anorthite (a simple model of basalt), with a eutectic near 1274 °C at roughly 42% anorthite. Melts on the diopside-rich side crystallize diopside first; melts on the anorthite-rich side crystallize anorthite first; both end at the eutectic.

The Al₂SiO₅ pressure–temperature diagram. For a single composition, a phase diagram can plot pressure against temperature instead. The Al₂SiO₅ polymorphs are the standard example, and they are why kyanite, andalusite and sillimanite are pressure and temperature indicators in metamorphic rocks.

In words: andalusite = low pressure; kyanite = high pressure at relatively low temperature; sillimanite = high temperature (at any crustal pressure). Density explains the pressure order (kyanite 3.6 > sillimanite 3.24 > andalusite 3.15).

Crystal systems (Division C)

How the system controls cleavage and shape. Symmetry repeats every property, including weak planes. In a cubic mineral, a cleavage plane in one direction must be repeated in the symmetrically equivalent directions, which is why cubic minerals cleave in 3 (halite, galena: cube), 4 (fluorite, diamond: octahedron) or 6 (sphalerite: dodecahedron) equal directions, or not at all (garnet). Lower-symmetry minerals have cleavages that are not repeated: one basal cleavage in micas and topaz, or two prismatic cleavages in pyroxenes. Calcite's threefold axis repeats its one rhombohedral cleavage in three directions at the same angles.

How the system controls optics. In cubic minerals, light travels at the same speed in every direction (isotropic), so they show no double refraction. Tetragonal, hexagonal and trigonal minerals are uniaxial (one direction, along c, with no double refraction); orthorhombic, monoclinic and triclinic minerals are biaxial. This is why garnet stays black in crossed polarizers while quartz and calcite do not.

Bravais lattices. A lattice can have points only at the corners of its unit cell, or extra points at its center or on its faces. In 1848 Auguste Bravais showed that the seven systems combined with these centring types give exactly 14 distinct three-dimensional lattices.

Some combinations are missing because they reduce to another lattice: a body-centered monoclinic cell, for example, can be redrawn as a base-centered one. Gold, silver and copper are face-centered cubic (F); calcite and corundum are rhombohedral (R); quartz is trigonal on the hexagonal P lattice.

Mineral terms glossary

Specialist words used in the mineral chapters and reference tables, in alphabetical order. Habit words on the rules' list of 21 are defined in Mineral habit and crystal form; the descriptive words below are extra.

How minerals form, ores, uses and gems

This chapter covers the rules' formation, ore and use topics: the six methods of formation, the minerals typical of each setting, ore deposits, uses by industry and gemstones.

How minerals form

Every crystal starts by nucleation: a few atoms assemble into a stable, ordered cluster (a nucleus), often on a dust grain, bubble wall or existing crystal, and the crystal then grows by adding atoms to its faces. Strong supersaturation or fast cooling makes many nuclei and many small crystals; slow cooling makes few nuclei that grow large.

Minerals typical of each rock-forming setting. The rules ask for minerals of each setting; the table pairs them with the rocks covered in the rock chapters.

Diamonds deserve a note because they appear as both a mineral and an ore question. They crystallize in the lithospheric mantle beneath old, cold continental cores (cratons) at depths of about 150 to 200 km. They reach the surface only in kimberlite (and the related lamproite) pipes: carrot-shaped volcanic conduits of volatile-rich ultramafic magma that rise fast enough that the diamonds do not convert to graphite on the way. Prospectors trace pipes using indicator minerals that travel with the diamonds and are easier to find in stream sediments: Cr-rich pyrope garnet, chrome diopside, Mg-ilmenite and chromite. Erosion of pipes also concentrates diamonds in placer gravels. In the United States, diamonds have been mined from a lamproite pipe at Crater of Diamonds State Park, Arkansas, the only diamond site in the world open to the public. More generally, an indicator mineral is any mineral whose presence points to a particular setting or process: kimberlite indicators for diamond pipes, index minerals for metamorphic grade, evaporite minerals for an arid, restricted basin.

Ore deposits

An ore is a rock or mineral from which a metal or other commodity can be extracted at a profit. Ore deposits are places where geological processes have concentrated an element far above its crustal average, and each process favors particular minerals.

Stratiform deposits are ore bodies that form a layer parallel to the bedding of their host sedimentary or volcanic sequence (BIF, many VMS and sediment-hosted lead–zinc or copper deposits, Bushveld chromite layers), unlike cross-cutting veins and pipes.

Uses of minerals by industry

The rules list ores, industry, jewelry, geochronology, medicine, manufacturing, construction and electronics. Tests ask for a specific use of a specific mineral, often the one that is not an ore use. Uses of rocks (building stone, aggregate, cement, coal) are covered in Economic uses of rocks; individual specimens' uses are in the Mineral reference tables.

Gemstones and lapidary terms

The rules name a closed list of gem varieties plus the gemstone minerals on the National List. The Gem varieties table gives each variety's parent mineral, formula, color cause and hardness. The concepts below are what tests ask about them.

Treatments, synthetics and simulants. Most gems on the market are treated, and tests occasionally ask which treatment produces a variety. Heating turns amethyst yellow-orange (most citrine on the market is heated amethyst, at roughly 400 to 500 °C), makes greenish aquamarine bluer, and improves the color and clarity of ruby and sapphire. Irradiation (often followed by heating) produces blue topaz and can darken quartz to smoky. Emeralds are commonly oiled or resin-filled to hide fractures, turquoise is stabilized with resin, and agate is often dyed. A synthetic gem has the same chemistry and structure as the natural one but was made in a laboratory (synthetic ruby has been made since the early 1900s by flame fusion); it fails the "naturally occurring" test, so it is not a mineral. A simulant only imitates the look with a different material: cubic zirconia, moissanite or glass imitating diamond. "Herkimer diamonds" (quartz) and "Matara diamonds" (zircon) are misleading trade names for natural stones, not diamonds and not simulants.

Organic gems. Pearl (nacre: aragonite tablets bound by protein, made by mollusks), mother-of-pearl (the nacre lining of shells), amber (fossilized tree resin, light enough to float in salt water, sometimes with trapped insects), jet (a hard, polishable variety of lignite coal) and coral are gems but not minerals, because living organisms made them.

What is a rock?

Rock or mineral?

A mineral is a naturally occurring, inorganic, solid, crystalline substance with a definite (but sometimes variable) chemical composition (see Mineral identification). A rock is an aggregate. Three consequences of the 2027 definition are favorite true/false items:

• A rock can be monomineralic (one mineral: quartzite, marble, rock salt, rock gypsum, dunite) or polymineralic (several: granite is quartz + K-feldspar + plagioclase + mica).
• A rock can consist of mineraloid matter, such as volcanic glass (obsidian, pumice) or opal-rich diatomite.
• A rock can be organic: coal is compressed plant material, and biochemical rocks such as coquina and chalk are built from shells and skeletons.

"All rocks are made of more than one mineral", "all rocks contain minerals" and "rocks are always crystalline" are all false. Granite "is a rock because it is made of several minerals" is the expected reasoning for a polymineralic example, but the deeper criterion is that it is a natural aggregate with no single formula.

The scientist who studies the origin, composition and texture of rocks is a petrologist. A mass of bedrock exposed at the surface is an outcrop. The word igneous comes from Latin ignis, fire.

The three rock families

Every family can become every other through the rock cycle: weathering and erosion (by water, wind, ice and gravity) break any rock into sediment, burial and heating metamorphose any rock, and melting resets any rock to magma (see The rock cycle and uses of rocks and minerals and Weathering and sediment).

When observation is not enough, the rules promise extra diagnostics (for example the mineral makeup of a fine-grained igneous rock, or the acid reaction of chalk versus diatomite). Use the rock reference tables for the specimen-by-specimen look-alikes.

Abundance facts that tests ask

Igneous rocks

Igneous rocks form when molten rock cools and solidifies. Underground melt is magma; melt erupted onto the surface is lava. Magma is a mixture of melt (mostly O, Si, Al, Fe, Ca, Mg, Na and K, the eight most common elements), suspended crystals and dissolved gases (mainly H₂O, then CO₂ and SO₂). Two things control what you see in the finished rock: how fast it cooled (texture) and what it was made of (composition).

Texture: how it cooled

Why cooling rate sets crystal size

Crystals start at nuclei, tiny ordered clusters of atoms, and then grow as ions diffuse to their surfaces. The cooling rate controls the balance between making new nuclei and growing old ones:

• Slow cooling (deep intrusion): the melt stays only slightly below its crystallization temperature, so few nuclei form and each has a long time to grow → a few large crystals (phaneritic).
• Fast cooling (lava flow): the melt is chilled far below its crystallization temperature, so many nuclei form at once and none has time to grow → many tiny crystals (aphanitic).
• Very fast cooling (quench, or very viscous melt in which atoms cannot move into position): no crystals form at all → glass.
• Slow cooling plus water gives the giant crystals of pegmatitic texture. Pegmatites crystallize from the last, water- and volatile-rich fraction of melt left inside a slowly cooling pluton (often squeezed into dikes and pods at its edges), so they get the long, insulated cooling of any intrusion. The fluid is the extra ingredient: water and other volatiles (F, B, Li) lower melt viscosity and speed up diffusion and suppress nucleation, so ions reach a few growing crystals easily and those crystals grow very large (centimeters to meters). A dry, viscous felsic lava is the easiest melt to quench to glass; a fluid basaltic lava crystallizes so readily that basaltic glass forms only as thin rinds on pillows or droplets.

Depth matters because of insulation: a pluton intrudes hot wall rock kilometers down and loses heat slowly. A thin dike or the edge of any intrusion against cold country rock cools faster, giving a fine-grained chilled margin that coarsens toward the center. Grain size within a lava flow follows the same rule: glassy or vesicular top and base, coarser interior.

Igneous texture terms

Crystal shape and crystallinity

These terms describe individual grains and the proportion of crystals to glass. They appear in matching sections and in thin-section questions.

The rule "early crystals are euhedral, late crystals are anhedral" lets you read the crystallization order straight off a rock: in granite, euhedral to subhedral biotite and plagioclase formed before anhedral quartz filled the remaining space, exactly as Bowen's series predicts.

Vesicles, amygdules and other features

• Vesicles are gas bubbles frozen in lava. They concentrate in the upper part of a flow because bubbles rise through the liquid before it solidifies; the flow top is often rubbly and vesicular and the interior denser. Vesicle size and abundance record how much gas escaped and how fast: many large, coalesced vesicles mean abundant, rapidly released gas.
• Amygdules are vesicles later filled with minerals (quartz, chalcedony/agate, calcite, zeolites, even native copper in Michigan's Keweenaw basalts) precipitated from fluids. A rock with them is amygdaloidal. Vesicles are empty holes; amygdules are filled holes. Neither is "country rock broken off by magma" (that is a xenolith).
• Xenolith ("foreign rock"): a fragment of country rock enclosed in an igneous rock. Mantle xenoliths of peridotite carried up in basalts and kimberlites are direct samples of the mantle.
• Joints are cracks along which the rock has split but not slid (a fault has movement). Massive igneous rock develops sets of evenly spaced joints as it cools and contracts, and later as erosion removes the overburden and the rock expands (sheet joints, see exfoliation). They are planes of weakness that guide weathering, erosion and quarrying.
• Columnar jointing: polygonal (often hexagonal) columns formed as a thick lava flow or sill contracts while cooling, cracks growing perpendicular to the cooling surfaces. Most common in basalt (Giant's Causeway, Northern Ireland; Devils Postpile, California).
• Pillow lava: rounded, bulbous pillows with glassy rinds, formed when lava erupts under water (mid-ocean ridges, the submarine flanks of Hawaii, ophiolites).
• Flow banding: streaky layering in viscous rhyolite and obsidian lavas.
• Spherulites and snowflake obsidian: volcanic glass is unstable and slowly devitrifies (crystallizes) into radiating clusters of cristobalite and feldspar, the white "snowflakes". For this reason obsidian older than about 20 million years (the Miocene) is rare; old glass also absorbs water and becomes perlite.

Composition: what it is made of

Felsic to ultramafic

The word felsic combines feldspar and silica; mafic combines magnesium and ferric (Latin ferrum, iron). Felsic minerals (quartz, K-feldspar, Na-plagioclase, muscovite, and feldspathoids: silica-poor, feldspar-like minerals such as nepheline, leucite and sodalite that form instead of feldspar when silica runs short; see Silica saturation) are light-colored and less dense (SG about 2.6–2.8); mafic (ferromagnesian) minerals (olivine, pyroxene, amphibole, biotite) are dark and denser (SG about 3.0–3.5) because they contain heavy iron. That single chemical fact explains why silica content, color and density all track together.

The percentages are textbook class boundaries; sources draw them a few percent apart (52 or 55, 63 or 65), so learn the order and approximate numbers rather than one exact line. Ordered from lowest to highest "heavy element" (Fe, Mg) content: felsic, intermediate, mafic, ultramafic. Ordered by silica in the common volcanic rocks: basalt < andesite < rhyolite.

Which elements dominate? In almost every igneous rock the two most abundant elements are oxygen (about 43–48% by weight) and silicon, because nearly all the minerals are silicates (in olivine-rich ultramafic rocks magnesium can overtake silicon); the classes differ in what comes next. A felsic rock is, after O and Si, richest in Al, K and Na (housed in quartz, K-feldspar, Na-plagioclase and muscovite) and poor in Fe, Mg and Ca. A mafic rock has a little less Si and much more Fe, Mg and Ca (plus plenty of Al in its Ca-plagioclase), all housed in olivine, pyroxene and Ca-plagioclase, and is poor in K and Na. Iron and magnesium are why mafic rocks are dark and dense.

A useful check value: a typical mid-ocean-ridge basalt contains about 50% SiO₂ and about 8% MgO. A "melt" with 50% SiO₂ is mafic; one with less than 45% is ultramafic.

Color index

The color index (CI, or M for "mafic") is the volume percentage of dark (mafic) minerals in the rock. It is the quickest field estimate of composition in a coarse-grained rock.

Working scale (used everywhere in this guide): felsic ≈0–15, intermediate ≈15–45, mafic ≈45–85, ultramafic >85.

IUGS vocabulary (a separate scale): hololeucocratic 0–10 · leucocratic 10–35 · mesocratic 35–65 · melanocratic 65–90 · holomelanocratic (ultramafic) 90–100.

The two scales answer different questions. The felsic-to-ultramafic names describe composition, and textbooks attach approximate CI ranges to them; the IUGS words (Greek leukos, white; melas, black) describe only how dark the rock is, with their own boundaries. The boundaries do not line up: a granite (CI ≈0–15) is hololeucocratic to leucocratic, and a gabbro with CI 60 is mafic but mesocratic. Answer in the vocabulary the question uses. "Light-colored" in a matching question means leucocratic, and the order leuco- → meso- → melano- is what is always tested.

Reading the mineral-percentage chart

The standard classification chart puts silica on the horizontal axis (or the felsic → ultramafic scale) and volume percent of each mineral on the vertical axis. Rock names run across the top, with the volcanic and plutonic name for each composition. To use it, draw a vertical line at the rock's composition and measure how much of the line each colored field occupies. To go backwards (mineral percentages → name), find the vertical line on which the proportions match.

Patterns to read off the chart:

• Quartz and K-feldspar appear only on the felsic side; olivine only on the mafic–ultramafic side. They never overlap, which is why olivine and quartz do not occur together in an igneous rock.
• Plagioclase spans the middle, changing from Na-rich (felsic side) to Ca-rich (mafic side), mirroring Bowen's continuous branch.
• Amphibole and biotite peak in intermediate rocks; pyroxene dominates mafic rocks.
• Granodiorite and dacite sit between granite/rhyolite and diorite/andesite. They are not on the ID list; they appear only on charts.

Naming a rock: the classification grid

Every common name is a texture × composition pair. This is the guide's igneous classification grid (each rock's full properties are in the Igneous rock table). Read down for texture, across for composition; SiO₂ and color-index boundaries are approximate. Names marked "not on the ID list" appear only for comparison.

Extrusive ↔ intrusive equivalents to memorize: rhyolite ↔ granite, andesite ↔ diorite, basalt ↔ gabbro, trachyte ↔ syenite, komatiite ↔ peridotite (trachyte and komatiite are not on the ID list). The hypabyssal equivalent of basalt/gabbro is diabase (dolerite), common in dikes and sills.

Quartz content separates the felsic-looking coarse rocks: granite has abundant quartz (≥20% of the light minerals); syenite looks like granite but has under about 5% quartz; diorite is plagioclase-dominated with ≤5% quartz and abundant hornblende/biotite. Of the National List igneous rocks, only granite, rhyolite and pegmatite (and their glassy/frothy relatives) typically carry more than 5% visible quartz; andesite, syenite, basalt, gabbro and peridotite do not.

Silica saturation

Whether a magma has enough silica to make quartz is a separate axis from felsic–mafic, and it explains several "which minerals cannot coexist" questions.

Quartz and feldspathoids never occur together: they would react to make feldspar (for example nepheline + 2 quartz → albite). Likewise Mg-rich olivine and quartz react to orthopyroxene (Mg₂SiO₄ + SiO₂ → 2 MgSiO₃), so olivine and quartz are not stable together. Feldspar without feldspathoids in a rock shows it is saturated or oversaturated.

The QAPF diagram (Division C)

The QAPF or Streckeisen diagram (IUGS) classifies coarse-grained (plutonic) rocks by the volume of four light-mineral groups: Q = quartz, A = alkali feldspar (orthoclase, microcline, sanidine, albite), P = plagioclase (An₅–An₁₀₀), F = feldspathoids (foids: nepheline, leucite, sodalite). Mafic minerals are ignored; the four are recalculated to 100%. Because quartz and feldspathoids cannot coexist, a rock plots in either the upper (Q) triangle or the lower (F) triangle, never both.

It is used for plutonic rocks because you must be able to see and count the minerals (the mode). Fine-grained and glassy volcanic rocks are instead classified chemically by the TAS (total alkali–silica: Na₂O + K₂O versus SiO₂) diagram. Ultramafic rocks (almost all dark minerals) are classified separately by olivine and pyroxene content.

The Bowen reaction series

The two branches

Discontinuous branch. As a basaltic melt cools, olivine crystallizes first. If the crystals stay in contact with the melt, they react with it at a lower temperature to form the next mineral: olivine + silica from the melt → pyroxene; pyroxene + melt (with water) → amphibole; amphibole → biotite. Each step is an abrupt change to a new mineral with a different silicate structure, hence "discontinuous". When the reaction is incomplete, a crystal keeps a core of the earlier mineral inside a reaction rim (corona) of the later one, for example pyroxene rimming olivine. Amphibole and biotite are hydrous (contain OH), so they can form only if the melt holds water.

Continuous branch. Plagioclase crystallizes over the whole temperature range. The first crystals are calcium-rich (anorthite, CaAl₂Si₂O₈); as the melt loses calcium and the temperature falls, the crystals and melt continuously exchange ions and the plagioclase becomes progressively sodium-rich (through bytownite, labradorite, andesine and oligoclase to albite, NaAlSi₃O₈). The mineral and its framework structure stay the same; only the composition shifts, by the coupled substitution Ca²⁺ + Al³⁺ ↔ Na⁺ + Si⁴⁺ (see Solid solutions, ternary diagrams and phase diagrams (Division C)). Na-rich plagioclase therefore contains more silica than Ca-rich plagioclase. If cooling is too fast for the exchange, crystals keep Ca-rich cores and Na-rich rims (normal zoning).

The merged tail. In the last, silica-, potassium- and water-rich melt, K-feldspar (orthoclase, microcline), then muscovite, then quartz crystallize. Quartz forms last because silica becomes concentrated only after the earlier minerals have removed Fe, Mg and Ca.

Temperatures are approximate and depend on composition and water content; tests reward the order and the relative temperatures. A test that asks "at which temperature would minerals contain the most calcium?" wants the highest temperature offered.

Why this order?

• Structure. High temperature favours simple, loosely linked structures (isolated SiO₄ tetrahedra in olivine). As the melt cools, structures share more oxygens: single chains, double chains, sheets, and finally the fully linked frameworks of feldspar and quartz. The degree of polymerization rises down the series.
• Chemistry. Early minerals use up Fe, Mg and Ca, which form high-melting compounds with little silica. What is left is enriched in Si, Na, K, Al and water.
• Melting is the mirror image. Minerals that crystallize last melt first. When a rock is heated, quartz, K-feldspar and Na-plagioclase begin to melt before olivine, which is why partial melting makes melts more felsic than their source (see Evolution of igneous melts (Division C)).

What Bowen's series explains

1. Which minerals occur together. Minerals that crystallize at similar temperatures are found together; minerals far apart on the series rarely coexist. Olivine + pyroxene + Ca-plagioclase → gabbro/basalt; amphibole + biotite + intermediate plagioclase → diorite/andesite; quartz + K-feldspar + Na-plagioclase + muscovite → granite/rhyolite. Olivine with quartz, or pyroxene with quartz and muscovite in a granite, is the classic "wrong pair".
2. Why granite has no olivine. Granite crystallizes from a residual (or crustally derived) melt whose Fe and Mg have already been used up, at temperatures where olivine is no longer stable. Any early olivine would have reacted to pyroxene and amphibole or been removed.
3. How one magma makes many rocks. Removing early crystals (fractional crystallization) leaves a melt that is progressively more felsic: basaltic → andesitic → rhyolitic. Granite, rhyolite and pegmatite are "residual-melt" rocks.
4. Weathering order. Minerals that formed at the highest temperatures are least stable at Earth's surface (25 °C, 1 bar), so olivine weathers fastest and quartz slowest. This is the Goldich dissolution series (see Weathering and sediment), which is why quartz dominates sand and olivine rarely survives transport.
5. Reading a rock's history. Phenocryst minerals tell you where crystallization had reached when the magma erupted: olivine phenocrysts in a porphyritic basalt; quartz phenocrysts in a rhyolite.

Magma and volcanism

Magma composition and properties

A magma's behavior (viscosity, gas content, eruption style) is controlled mainly by its composition (especially silica and dissolved volatiles) and its temperature.

Ultramafic komatiite lava erupted at about 1600 °C, almost entirely in the Archean, when the mantle was hotter; essentially none forms today. The rule "magmas that form at lower temperatures and pressures tend to be high in silica and viscous; magmas that form at high temperatures tend to be low in silica and fluid" follows directly from Bowen's series.

Viscosity, gas and eruption style

Why silica raises viscosity. In a melt, SiO₄ tetrahedra link through shared oxygens into chains and networks. The more silica, the more extensive these polymerized networks, and the harder it is for the melt to flow. Fe, Mg and Ca break up the networks, so mafic melts are runny. Higher temperature breaks bonds and lowers viscosity; dissolved water also depolymerizes the melt. Going down Bowen's series, residual melt becomes more silica-rich and more viscous.

Why gas makes eruptions explosive. Magma at depth holds water and CO₂ in solution, like CO₂ in a sealed soda. As magma rises, pressure falls, the gas exsolves into bubbles, and the bubbles expand. In a low-viscosity basalt the bubbles rise and escape: fire fountains and lava flows (effusive). In a high-viscosity rhyolite the bubbles cannot escape; pressure builds until the foam fragments into pumice and ash (explosive). The primary cause of a pyroclastic eruption is the exsolution of dissolved gas from viscous magma. A felsic magma that has already lost its gas erupts as a slow, sticky lava dome (high viscosity, low gas).

Eruption styles in brief (named after type volcanoes):

• Hawaiian: fluid basalt; lava fountains and flows (Kilauea, Mauna Loa).
• Strombolian: basaltic; short gas bursts throw out cinders and bombs and build cinder cones (Stromboli, Parícutin).
• Vulcanian: andesitic; short, violent blasts clear a plugged vent (blocks, ash).
• Peléan: a collapsing lava dome sends a pyroclastic flow (nuée ardente, "glowing cloud") downslope (Mont Pelée 1902).
• Plinian: gas-rich, viscous dacitic–rhyolitic magma; a sustained, hot, buoyant column tens of kilometers high rains pumice and collapses into pyroclastic flows (Vesuvius AD 79, Mount St. Helens 1980, Pinatubo 1991).

Volcanic gas is mostly water vapor, then CO₂ and SO₂; SO₂ makes stratospheric sulfate aerosols that cool the climate for a year or two (Pinatubo 1991: about 0.5 °C).

Volcano types

Lava and pyroclastic material

Tephra (pyroclasts, pyroclastic material) is the general name for all fragments ejected by an eruption. Pyroclastic flows (nuées ardentes) are ground-hugging avalanches of hot gas, ash, pumice and lava blocks moving at more than 100 km/h (up to several hundred km/h) and several hundred degrees Celsius, driven by gravity. When they come to rest the deposit is ignimbrite; if hot enough the fragments fuse into welded tuff, with pumice flattened into dark lenses. Tuff is consolidated ash (classified further by setting: subaerial, submarine, lacustrine, reworked); tuff breccia has 25–75% blocks over 64 mm in ash. Pumice is light enough to float; the 1883 Krakatau eruption produced pumice rafts that carried debris across the Indian Ocean.

Where igneous rocks form

Intrusive and extrusive bodies

Molten rock that solidifies underground forms an intrusion (pluton); the older rock it invades is country rock (wall rock). Intrusions are later exposed by uplift and erosion, often standing up as hills because igneous rock resists erosion better than the surrounding sediment.

How to identify a sill from a buried lava flow: a sill bakes (contact-metamorphoses) the rock both above and below it and has chilled margins on both sides, while a lava flow bakes only the rock beneath it and has a weathered, vesicular top that later sediment rests on (see Dating rocks and minerals for cross-cutting relationships). Devils Tower (Wyoming) is a shallow (hypabyssal) intrusion of phonolite porphyry with spectacular columnar joints, exposed by erosion. A landscape where a once-low intrusion or lava now stands as a ridge because softer surrounding rock eroded away shows inverted topography.

How rocks melt

Most of the mantle is solid. The geotherm is the actual temperature at each depth (temperature rises with depth); the solidus is the temperature at which a rock first begins to melt at that depth (pressure); the liquidus is the temperature at which it is fully molten. Between them the rock is partly molten. Rock melts only where the geotherm crosses the solidus, which normally does not happen, so magma needs one of three triggers:

Ranked by the volume of magma produced: decompression melting at ridges (by far the most; it makes all the oceanic crust) > flux melting at subduction zones > heat-transfer and hot-spot melting. To melt more rock, raise the temperature, lower the pressure, or lower the melting point (add water). Adding water lowers both the solidus and the liquidus.

Plate-tectonic settings of magma

Why subduction magma becomes andesitic: the mantle wedge first makes a basaltic melt; as it rises through 30–70 km of crust it crystallizes early mafic minerals (fractional crystallization), absorbs crustal rock (assimilation) and mixes with crustal melts (magma mixing), all of which push it toward intermediate and felsic compositions. Felsic rocks (granite, rhyolite) therefore characterize continental plate boundaries, and mafic rocks (basalt, gabbro) characterize oceanic crust and divergent boundaries. In a zoned arc magma chamber, the outer (wall) rock that crystallized first is often more mafic and the core more felsic, or the reverse if felsic melt rises to the roof: questions about "lithology from wall to center" test this logic.

Hot spots and mantle plumes

A hot spot is a long-lived area of volcanism fed by a mantle plume, a column of unusually hot mantle rising from deep in the mantle (possibly the core–mantle boundary). Its head melts by decompression. Hot spots are largely independent of plate boundaries and stay roughly fixed while plates move over them, so they leave age-progressive volcanic chains.

• Hawaii (oceanic plate): basaltic shield volcanoes (Mauna Loa, Kilauea) above the plume; the islands get older to the northwest (Kauaʻi about 5 million years), continuing as the Hawaiian–Emperor seamount chain. Young submarine volcano Kamaʻehuakanaloa (formerly Lōʻihi) is forming to the southeast. Rocks: basalt, scoria, olivine-bearing (peridot) basalt; pillow basalt on the submarine flanks.
• Iceland: a hot spot sitting on the Mid-Atlantic Ridge, so both decompression sources add up; extensive basaltic lava flows and fissure eruptions, with some rhyolite.
• Yellowstone (continental plate): basaltic plume magma melts the continental crust, so eruptions are bimodal (basalt and rhyolite). Huge caldera-forming eruptions about 2.1, 1.3 and 0.63 million years ago produced welded tuffs, obsidian (Obsidian Cliff) and rhyolite flows. The Snake River Plain marks the track, older toward the southwest.
• Flood basalts (large igneous provinces) such as the Columbia River Plateau (mainly about 17–15 million years ago), the Deccan Traps (about 66 million years ago) and the Siberian Traps (about 252 million years ago) are attributed to the arrival of plume heads: enormous volumes of fluid basalt erupted from fissures in a short time.

Ophiolites and the structure of oceanic lithosphere

Oceanic lithosphere has a layered structure made at mid-ocean ridges. From the top down:

1. Deep-sea sediment (chert, shale, limestone) settled on the seafloor.
2. Pillow basalts: lava erupted onto the seafloor.
3. Sheeted dike complex: vertical diabase dikes, side by side, that fed the lavas as the plates pulled apart.
4. Gabbro (often layered cumulate gabbro at its base): the crystallized magma chamber beneath the ridge.
5. Peridotite (mantle): the depleted mantle left after basalt was extracted; commonly altered by seawater to serpentinite.

An ophiolite is a slice of this oceanic crust and upper mantle that has been thrust (obducted) onto a continent at a convergent boundary. Ophiolites are where peridotite and serpentinite reach the surface (California's Coast Range ophiolite, the Troodos ophiolite of Cyprus, the Semail ophiolite of Oman, Bay of Islands in Newfoundland) and host chromite, copper sulfide and asbestos deposits. An outcrop that runs peridotite, gabbro, sheeted dikes, pillow basalt and sediment is an ophiolite: ancient seafloor uplifted in a collision.

Kimberlites and the source of diamonds

Kimberlite is a volatile-rich (CO₂, H₂O) ultramafic rock, broadly peridotite-like in composition, that erupts extremely rapidly from depths of more than about 150 km through ancient, thick continental cratons. It forms carrot-shaped pipes (diatremes) filled with broken kimberlite and mantle xenoliths. Because diamond is stable only at the high pressures of those depths, kimberlite pipes are the primary source of diamonds; the fast ascent brings diamonds up before they can convert to graphite. Prospectors trace pipes using resistant indicator minerals weathered out into stream gravels: Cr-rich pyrope garnet, chrome diopside, magnesian ilmenite and chromite. Kimberley (South Africa) gave the rock its name.

Evolution of igneous melts (Division C)

Partial melting

Rocks are mixtures of minerals with different melting temperatures, so they melt over a range: the lowest-melting fraction (quartz, K-feldspar, Na-plagioclase and the incompatible elements) melts first. The melt is separated (squeezed out, rising buoyantly) from the solid residue, a process called partial melting. Consequences:

• Partial melting of mantle peridotite (typically 10–20% melting at ridges) yields basalt, not peridotite, because only the lower-melting pyroxene and plagioclase-forming components melt; the olivine-rich residue stays behind as depleted peridotite.
• The smaller the degree of melting, the more enriched the melt is in incompatible elements and alkalis (Na, K). Very small degrees of melting make alkaline melts (alkali basalts, nepheline-bearing rocks), typical of continental rifts and the late stages of hot spots. Volatiles help: water and CO₂ (for example recycled from the surface by subduction, or carried by fluids that have metasomatized, i.e. chemically enriched, the mantle) lower the solidus, so small, alkali-rich melts form more easily. Large degrees of melting make tholeiitic basalt (mid-ocean ridges).
• Melting of sedimentary and metamorphic rocks also produces magma (true): melting of continental crust gives granitic magma.

Fractional crystallization and crystal settling

Fractional crystallization is crystallization in which early-formed crystals are separated from the remaining melt so they cannot react with it (contrast with equilibrium crystallization, where crystals keep reacting along Bowen's series). The separated crystals take the elements they need with them, and the remaining melt changes composition step by step.

How the crystals are removed in nature:

• Crystal settling: dense early minerals (olivine about 3.3, pyroxene, chromite) sink to the floor of the chamber. Light late minerals (plagioclase, especially in iron-rich melts) may float toward the roof.
• Filter pressing: melt is squeezed out of a crystal mush.
• Flow segregation and crystallization on the cool walls of the chamber.

Results:

• The melt evolves from basaltic (≈50% SiO₂) toward andesitic and rhyolitic; "after most minerals have crystallized, the remaining melt contains more silica than the original". Mg is removed before Fe (early olivine and pyroxene are Mg-rich), so in tholeiitic magmas the melt becomes iron-enriched.
• The last melt is enriched in everything the early minerals rejected: silica, aluminum, water and above all the alkali elements potassium and sodium. Olivine, pyroxene and Ca-plagioclase lock up Mg, Fe and Ca but take in almost no K and little Na, so these build up in the shrinking liquid. That is why the final minerals to crystallize are the K-, Na-, Al- and Si-rich ones at the foot of Bowen's series, K-feldspar, muscovite and quartz (with Na-rich plagioclase), and why granite and pegmatite are the typical residual-melt rocks.
• The accumulated crystals form cumulate rocks: layers of nearly pure olivine (dunite), chromite, pyroxene or plagioclase (anorthosite) on the chamber floor. Layered mafic intrusions such as the Bushveld Complex (South Africa, the world's main chromium and platinum-group-metal source), the Stillwater Complex (Montana) and the Skaergaard intrusion (Greenland) show this layering like sedimentary beds.
• The Palisades Sill has an olivine-rich layer near its base: olivine crystallized first and, being denser than plagioclase and the melt, settled to the bottom; plagioclase-rich layers lie above it.

Assimilation

Hot magma rising through the crust melts, digests and absorbs the surrounding country rock (assimilation). Blocks pried off the chamber roof sink into the magma (stoping). Undigested blocks remain as xenoliths. Because continental crust is felsic and melts at lower temperature than basalt, assimilation usually pushes a basaltic magma toward intermediate or felsic composition (a mafic melt intruding continental crust will melt or assimilate it). Assimilation consumes heat, so it is usually coupled with fractional crystallization (AFC).

Magma mixing

Two magmas of different composition meet in a chamber or conduit (for example basalt injected into the base of a rhyolitic chamber) and blend. Thorough mixing gives an intermediate (often andesitic) magma; incomplete mixing (mingling) leaves dark, rounded mafic enclaves in lighter rock or swirled banding. Mixing often triggers eruptions, because the hot mafic magma heats the felsic one and adds gas.

All three of fractional crystallization, assimilation and magma mixing can raise the silica content of a basaltic magma (answer "all of the above"). Over time a typical magma body becomes more felsic before it erupts.

Zoning and disequilibrium textures

Incompatible elements and pegmatites

Incompatible elements are elements whose ions are the wrong size (too large: K⁺, Rb⁺, Cs⁺, Ba²⁺, the light rare earths) or the wrong charge (too highly charged: Zr⁴⁺, Nb⁵⁺, Ta⁵⁺, U⁴⁺, Th⁴⁺), or simply too rare and too small (Li⁺, Be²⁺, B³⁺), to substitute into the crystal sites of olivine, pyroxene, amphibole and plagioclase. As a magma crystallizes, the common minerals reject them, so their concentration in the shrinking liquid keeps rising. The last fraction of a granitic magma is therefore rich in water, other volatiles (F, Cl, B) and incompatible elements.

That late fluid crystallizes as pegmatite, usually as dikes, veins and pods in and around granite batholiths:

• Why the crystals are huge: water lowers the viscosity of the melt and lets ions diffuse quickly to a few growing crystals, while nucleation is slow; crystals of feldspar, quartz, mica and spodumene reach meters.
• Why pegmatites are mineral treasure troves: the incompatible elements finally reach concentrations high enough to build their own minerals.

Because pegmatites crystallize from the most evolved granitic melt, they are usually felsic (granitic) and leucocratic, and a pegmatite is a sign that large, high-quality crystals and gem minerals may be nearby. For the gem names and properties of each mineral, see the mineral reference tables.

Weathering and sediment

Every sedimentary rock starts as rock that broke down at Earth's surface. Weathering is the breakdown of rock in place. Erosion is the removal and transport of the products by water, wind, ice or gravity, and deposition is where they come to rest. The two words are not synonyms: weathering makes the sediment, erosion moves it. Questions here ask you to name a weathering process from a description, to write or recognize the feldspar-to-clay reaction, to rank minerals by how fast they weather, and to explain why some rocks last longer than others.

Mechanical weathering

Mechanical weathering exploits weaknesses (joints, grain boundaries, bedding) and wedges rock apart. The products are the same minerals as the parent rock, just smaller: a granite broken by frost gives angular pieces of granite and loose grains of quartz, feldspar and mica.

Exfoliation of plutonic rock is a direct consequence of the rock cycle: the rock formed at high pressure and temperature, so it is out of equilibrium at the surface. The same mismatch explains chemical weathering below.

Chemical weathering

Chemical weathering converts minerals that formed at high temperature and pressure (or in the absence of air and water) into minerals stable at the surface: clay, iron oxides and dissolved ions. The key reactant is water, especially water made weakly acidic by dissolved carbon dioxide, which forms carbonic acid:

What happens to each mineral of a granite during chemical weathering is a favorite question:

The dissolved ions go to the sea. Calcium and bicarbonate are taken up by organisms to build shells (later limestone), dissolved silica is used by diatoms and radiolarians (later chert or diatomite), and sodium and chloride stay dissolved until seawater evaporates (rock salt).

Spheroidal weathering combines the two types: chemical attack works in from joint surfaces, fastest at corners (attacked from three sides) and edges (two sides), so angular joint blocks become rounded boulders that peel in concentric shells. It is common in granite and basalt.

Karst. Groundwater with carbonic acid dissolves soluble bedrock along joints, producing caves, sinkholes, disappearing streams and, in the humid tropics, tower karst (steep residual hills). Karst forms mainly in limestone and dolostone, and also in marble, chalk, gypsum and rock salt. Sinkholes form in two main ways. As acidic groundwater widens joints and bedding planes into caves, the rock or soil roof over a growing cavity eventually becomes too thin to hold itself up and collapses into it (a collapse sinkhole, which can open suddenly, often after the water table drops and no longer buoys the roof, or after heavy rain). Where limestone lies at or near the surface, slow dissolution from the top down instead makes a gentle bowl (a solution sinkhole). Florida (whose bedrock is almost entirely limestone and dolostone), Kentucky (Mammoth Cave) and the Yucatán are classic karst regions; land over karst is risky to build on, and pumping groundwater makes collapse more likely. When cave water loses CO₂ to the air, the carbonation reaction reverses and calcite is redeposited as stalactites, stalagmites and flowstone (the same process makes travertine).

Mineral stability: the Goldich dissolution series

In 1938 Samuel Goldich noticed that the order in which igneous minerals weather is the order in which they crystallize in Bowen's reaction series (see The Bowen reaction series). A mineral is most stable in conditions like those it formed in. Olivine and calcium plagioclase form at about 1,200 °C with no free water, so at 25 °C in rain they are far from equilibrium and weather fastest. Quartz forms last, at the lowest temperature, and is the most stable at the surface.

Read it side by side with Bowen's series: the same minerals in the same rows, with the arrow reversed.

A second reason works with the first: structure. Olivine is an isolated-tetrahedron silicate with Fe²⁺ and Mg²⁺ holding tetrahedra together, so breaking a few metal–oxygen bonds frees whole tetrahedra, and its iron oxidizes. Quartz is a framework of Si–O bonds in all three directions with no cations to remove. The more Si–O–Si linkage, the more resistant the mineral.

Consequences tested often:

• Mafic rocks (basalt, gabbro, peridotite) weather faster than felsic rocks (granite, rhyolite) because they are made of the high-temperature, iron- and magnesium-rich minerals. Iron-bearing mafic minerals also produce the most iron oxide when they weather.
• Quartz is the predominant mineral of beach and desert sand because everything else has been weathered away or ground down.
• A rock made only of quartz held by quartz cement (quartz sandstone with silica cement, quartzite, chert) is the most resistant common rock. Quartzite resists weathering better than its parent sandstone because its interlocking quartz has no pores or weak cement.
• An unweathered olivine-rich sand (as on Hawaii's green-sand beach) is immature: it can exist only very close to its source.

What controls weathering rates

• Climate. Chemical reactions speed up with temperature (roughly doubling for every 10 °C rise) and need water, so chemical weathering dominates in warm, humid climates (rainforest soils are deep and clay-rich, with kaolinite). Mechanical weathering dominates in cold climates (frost wedging) and deserts (salt growth, thermal stress). Arid climates have slow chemical weathering, which is why ancient monuments survive in Egypt.
• Surface area. Breaking a cube into eight smaller cubes doubles the surface exposed. Fractured, jointed or finely broken rock weathers far faster than a solid block.
• Rock type and mineral composition. Follow the Goldich series; soluble rocks (limestone, marble, rock salt, gypsum) dissolve; porous rocks let water in. Marble and limestone are dissolved by acid rain much faster than granite.
• Cement and structure. A sandstone with calcite cement crumbles when the cement dissolves; one with silica cement survives. Foliated rocks such as schist split and slide along their foliation, which makes schist slopes unstable after blasting.
• Time and exposure. Longer exposure gives more weathering; the ratio of altered to fresh minerals measures how long and how intensely a rock was weathered.
• Vegetation and organisms. Roots wedge rock and release CO₂ and organic acids; lichens etch surfaces.

Differential weathering: when a resistant layer (sandstone, quartzite, a dike of fresh basalt) sits next to a weak one (shale, limestone in a wet climate), the weak rock wears back and the resistant one stands up as a ridge, cap rock or hoodoo.

Soils and regolith

Regolith is all the loose material covering bedrock: weathered rock, sediment and soil. Soil is the upper part of regolith that has been altered enough, and mixed with organic matter, to support plants. A soil profile has horizons: O (organic litter), A (topsoil, humus plus minerals), E (leached zone, in some soils), B (subsoil, where clay, iron oxides and calcite washed down from above accumulate), C (partly weathered parent rock) and R (bedrock). Soil type is controlled by climate more than by parent rock.

Laterite is the end member of tropical weathering. Under heavy rain and high temperature, even silica is leached out, leaving a red soil rich in iron and aluminum oxides and hydroxides. Where the parent rock was rich in aluminum and poor in iron, the residue is bauxite, the aluminum ore (see Chemical and biochemical rocks).

Erosion, transport and sediment maturity

The agents of erosion are running water (by far the most important), wind, glacial ice, waves and currents, and gravity (mass wasting: landslides, rockfalls, slumps, creep). Each agent sorts sediment differently:

Competence is the largest grain a current can move; capacity is the total amount of sediment it can carry. Both rise steeply with velocity, which is why floods do most of a river's work (see Grain size and stream velocity). A stream also lengthens its valley upslope by headward erosion: water pouring into the head of the channel erodes it backward, away from the direction of flow.

Sediment maturity describes how far weathering and transport have processed a sediment. It is the key to reading where a sandstone came from.

• Compositional maturity: the proportion of stable grains (quartz, plus chert and quartzite fragments) to unstable ones (feldspar, rock fragments, mafic minerals). Quartz sandstone (quartz arenite, over 90% quartz) is compositionally mature; arkose (feldspar-rich) and greywacke (rock fragments) are immature.
• Textural maturity: how much clay matrix has been winnowed out and how well sorted and rounded the grains are. In order: immature (much clay matrix, poorly sorted, angular), submature (little clay, still poorly sorted), mature (no clay, well sorted, not yet rounded), supermature (no clay, well sorted, well rounded).

Maturity increases with transport distance, time and especially with reworking by waves and wind. A supermature quartz sandstone with frosted grains has often been through several cycles of erosion and deposition.

Sedimentary rocks

Sedimentary rocks form at or near Earth's surface, at low temperature and pressure, from sediment that is buried and lithified. They make up only about 5% of the crust by volume but about 75% of the rocks exposed at the surface. They are the only rocks that routinely contain fossils, and their layering (stratification, or bedding) is their most important single feature. The 2027 rules limit sedimentary textures to three: clastic (also called detrital or terrigenous), chemical (crystalline) and biochemical (organic or bioclastic). Per-specimen descriptions are in the rock reference tables; this chapter covers the concepts behind them.

Clastic, chemical and biochemical textures

* = State and National only. A bioclastic rock (coquina, fossil hash) is made of broken shell fragments, so it is biochemical in origin but clastic in texture; a coquina whose shells were broken and transported by waves is a calcarenite (a sand-size carbonate rock). Clastic rocks are far more abundant than chemical ones because weathering produces enormous volumes of sediment everywhere.

Grain size: the Wentworth scale

Clastic grain size is measured on the Udden–Wentworth scale. Each class boundary is double the one below it (powers of 2 mm), so the classes are evenly spaced on a logarithmic scale. Geologists also use Krumbein's phi (φ) scale, φ = −log₂(diameter in mm): 1 mm = 0 φ, 2 mm = −1 φ, 1/16 mm = 4 φ, 1/256 mm = 8 φ. Larger grains have smaller (more negative) phi numbers.

Gravel is the collective term for all grains over 2 mm. Mud is silt plus clay; mudstone is a blocky mud rock and shale is a mud rock that is fissile (splits into thin sheets along bedding because its platy clay minerals lie flat). Note that "clay" is both a size (under 1/256 mm) and a group of sheet-silicate minerals; most clay-size particles are clay minerals, but not all.

Sorting and rounding

Sorting is the range of grain sizes in a sediment. Well sorted means one size (one energy level acting steadily); poorly sorted means many sizes dumped together (energy dropped suddenly, or the agent carries everything at once). Geologists rank sorting as very well, well, moderately, poorly and very poorly sorted; engineers use the opposite words ("well graded" means poorly sorted).

Sorting also controls porosity and permeability: in well-sorted sediment the pores between grains stay open, while in poorly sorted sediment small grains fill the gaps. Coarse, well-sorted gravel is the most permeable common sediment, so water flows through it fastest (and contaminants spread fastest); clay is porous but nearly impermeable because its pores are tiny.

Roundness describes the sharpness of a grain's corners and edges, from very angular, angular, subangular, subrounded, rounded to well rounded. Sphericity is a different property: how close the overall shape is to a sphere (equant vs elongated or flat). A flat pebble can be well rounded but have low sphericity.

• Rounding happens by abrasion during transport: grains knock and grind against each other and the bed, so corners are worn off first. Angular grains have not traveled far (breccia at the foot of a cliff, glacial till, landslide debris); rounded grains have traveled far or been reworked for a long time (river gravel, beach and dune sand).
• Larger clasts round faster than small ones: a pebble rolling in a river is rounded within kilometers to tens of kilometers, but sand needs hundreds of kilometers of river transport (or long reworking by wind and waves), and silt hardly rounds at all because water cushions the impacts.
• Wind rounds sand more efficiently than water (no water cushion), and the impacts frost the surfaces, so well-rounded frosted quartz grains are the signature of desert dunes.
• Breccia vs conglomerate is the classic pair: both have clasts over 2 mm; breccia's are angular (little transport, deposited near the source, for example on an alluvial fan, at a cliff base, in a fault zone or a collapsed cave), conglomerate's are rounded (transported and abraded in a river or on a beach).

Grain size and stream velocity

The 2027 rules tie grain size directly to water velocity in rivers and streams. The standard tool is the Hjulström diagram (Hjulström curve), which plots grain size on the horizontal axis against flow velocity on the vertical axis, both on logarithmic scales. Two curves divide it into three fields.

• Erosion (upper curve). Above it, the flow can lift grains of that size off the bed. Fine to medium sand (about 0.1–0.5 mm) is the easiest sediment to erode, needing only roughly 20–30 cm/s. Coarser grains need more velocity because they are heavier: pebbles need roughly 1 m/s and cobbles several m/s.
• Clay and silt are hard to erode even though they are tiny. The flat platy particles stick together (cohesion) and form a smooth bed that the flow skims over, so the erosion curve rises again on the left of the diagram. A compacted clay bed can need as much velocity to erode as gravel.
• Transportation (between the curves). A grain that is already moving keeps moving at velocities well below the one needed to pick it up.
• Deposition (below the lower curve). When the flow slows below the settling curve, grains of that size drop out. For clay the settling velocity is so low that clay stays suspended in almost any moving water and settles only in still water such as lakes, lagoons, floodplain ponds and the deep sea.

The same logic explains sorting and environments: high-velocity mountain streams and alluvial fans leave gravel, rivers in flood leave sand in their channels and silt on the floodplain, and only quiet water lets clay settle into what becomes shale. When a stream's discharge is low, the largest clasts (cobbles and boulders) do not move at all; they move only in floods.

Lithification and diagenesis

Diagenesis is every physical, chemical and biological change that happens to sediment after deposition and before metamorphism, at temperatures up to roughly 150–200 °C and depths of a few kilometers. Lithification, turning loose sediment into rock, is the main part of diagenesis:

1. Compaction. The weight of overlying sediment squeezes grains closer together, expels water and reduces pore space. It is the dominant process for mud: a freshly deposited mud can be 60–80% water by volume and loses most of it, and the clay flakes rotate to lie flat, which gives shale its fissility.
2. Cementation. Groundwater moving through the pores precipitates minerals that glue grains together. It is the dominant process for sand and gravel, whose rigid grains compact little.
3. Recrystallization. Unstable minerals change into stable ones without changing much chemically: aragonite shells recrystallize to calcite, opal to chalcedony and quartz, lime mud to crystalline limestone. Other diagenetic changes include replacement (dolomitization, silicification of wood into petrified wood and of shells into chert) and authigenesis (new minerals, such as pyrite in organic-rich mud).

Matrix is fine sediment (mud) deposited together with the coarser grains; cement is precipitated later in the pores. Greywacke has a lot of matrix; a clean quartz sandstone has cement only.

Color as a clue. Red, purple and brown sedimentary rocks owe their color to ferric (Fe³⁺) oxides and usually formed in oxidizing, often terrestrial, settings. Gray, black and dark green rocks usually formed in reducing (oxygen-poor) settings, such as swamps, stagnant lagoons and deep basins, where organic carbon is preserved and iron stays ferrous or forms pyrite.

Clastic rocks

The sandstone family is a maturity series: greywacke (most immature, muddy and lithic) → arkose (feldspathic) → quartz sandstone (mature). Fossils are most common in shale and limestone, which form in quiet water where organisms live and remains are buried gently; they also occur in sandstone, but are rare in coarse conglomerate and breccia, where the energy breaks them.

Chemical and biochemical rocks

Carbonate rocks

Limestone is rock made mostly of calcite (CaCO₃). Most limestone is biochemical: corals, molluscs, brachiopods, crinoids, foraminifera and calcareous algae extract Ca²⁺ and HCO₃⁻ from seawater to build calcite or aragonite skeletons. Carbonate production is greatest in warm, clear, shallow tropical seas (the Bahamas, the Great Barrier Reef, the Persian Gulf today), away from rivers that would bury organisms in mud. Calcite is also less soluble in warm water, which helps.

Evaporites

Evaporites are chemical sediments left when a body of saline water evaporates faster than it is replenished: a restricted arm of the sea, a lagoon, a salt flat (sabkha) or a lake with no outlet in a hot, dry climate (the Dead Sea, Great Salt Lake). Minerals precipitate in reverse order of solubility, the least soluble first:

Because seawater holds far more NaCl than CaSO₄, evaporating 1,000 m of seawater gives only about 15 m of evaporite, most of it halite. Thick evaporite deposits therefore require a basin that was repeatedly refilled. Evaporite beds commonly show plane lamination, mud cracks, halite crystal casts (hopper crystals) and nodular gypsum.

Siliceous rocks: chert and diatomite

Chert is microcrystalline to cryptocrystalline quartz (chalcedony). It breaks with conchoidal fracture, is hard (about 7), and has a waxy to dull luster. Most chert is biochemical: the opal (amorphous silica) skeletons of radiolarians, diatoms and sponge spicules accumulate on the sea floor, dissolve and reprecipitate as microcrystalline quartz during diagenesis. Bedded chert forms in deep ocean basins below the depth where carbonate dissolves; nodular chert forms as lumps in limestone and chalk, where silica replaces carbonate. Flint is dark gray to black nodular chert, classically from chalk; jasper is red chert colored by hematite. Because it fractures to sharp edges, flint was the main toolmaking stone of the Stone Age.

Diatomite (State/National) is the unrecrystallized deposit of diatom frustules: white, chalk-like, extremely light and porous. Unlike chalk, it does not fizz, because it is opal (SiO₂·nH₂O), not calcite. It forms in lakes and in nutrient-rich ocean waters. Uses: filters (swimming pools, beer), mild abrasives, insulation and absorbents.

Coal

Coal is a biochemical (organic) sedimentary rock made mostly of carbon from land plants. The steps:

1. Accumulation. Plants grow and die in a swamp (or bog). The waterlogged, stagnant, anoxic (oxygen-poor), often acidic water stops bacteria and fungi from fully decomposing the plant matter, so it accumulates as peat.
2. Burial (coalification). Sediment buries the peat. Compaction squeezes out water, and heat drives off volatile matter (water, CO₂, methane), so the coal loses mostly hydrogen and oxygen and its proportion of carbon rises: peat → lignite → sub-bituminous → bituminous → anthracite.
3. Rank increases with depth, temperature and time. Anthracite forms only where coal was heated by deep burial or mountain building, so many geologists class it as a low-grade metamorphic rock.

(Carbon percentages are USGS ranges; ranks are formally defined by heat content and fixed carbon, so other sources give slightly different numbers.)

Most of the world's coal formed in the Carboniferous period (about 359–299 million years ago; in North America the coal-rich Pennsylvanian), when vast coal swamps of tree-sized ferns, horsetails and club mosses covered tropical lowlands. There is practically no coal older than the Devonian, because before then there were no large land plants. Moisture lowers coal's heating value, which is one reason anthracite and bituminous coal are worth more than lignite.

Coal versus oil: coal forms from land plants buried in swamps and stays where it formed as a solid rock; oil and gas form mainly from marine plankton and algae buried in organic-rich mud (later black shale, the source rock) and then migrate as fluids into porous reservoir rock.

Two ways textbooks group these rocks. The 2027 rules use three textures: clastic, chemical and biochemical, with coal counted as biochemical (organic). Some textbooks use four groups instead, splitting off organic rocks: biochemical then means rocks whose minerals were built by organisms (fossil limestone, coquina, chalk, diatomite, most chert), and organic means rocks made of the carbon-rich remains of organisms themselves (coal; also oil shale). Others use detrital / chemical (including biochemical limestone) / organic. The rock is the same in every scheme; answer with the scheme the question uses, and if it lists "organic" as an option, coal goes there.

Iron and aluminum rocks: banded iron formation and bauxite

Banded iron formation (BIF) consists of alternating layers of iron oxides (hematite, magnetite; metallic gray or red) and chert (red jasper or gray chert). The banding exists at three scales: macrobands several meters thick (separated by thin shale beds), mesobands of chert or iron oxide from millimeters to a few centimeters thick (the stripes you see in a specimen), and microbands under a millimeter thick within the chert mesobands. BIF is almost all Precambrian: mostly 2.6–1.8 Ga (oldest about 3.8 Ga; a brief return about 0.75 Ga). Origin: the early ocean was anoxic, so iron dissolved as Fe²⁺. When photosynthetic cyanobacteria began releasing oxygen, culminating in the Great Oxidation Event about 2.4 billion years ago, dissolved iron was oxidized to insoluble Fe³⁺ and precipitated as iron oxides, alternating with silica layers (seasonal or cyclic changes are the usual explanation for the banding). Once the deep ocean became oxygenated, dissolved iron disappeared and large BIFs stopped forming. BIFs are the world's main iron ore (Lake Superior region, Pilbara of Australia, Brazil).

Bauxite is a residual deposit, not a deposited sediment: it is what is left after intense chemical weathering in hot, wet, tropical climates leaches out silica, alkalis and much of the iron from an aluminum-rich parent rock (granite, shale, basalt, or clay over limestone). It is a mix of aluminum hydroxide minerals (gibbsite, boehmite, diaspore) with clay and iron oxides. In hand specimen it is earthy and dull, white, cream, tan, red-brown or orange (red from iron), often pisolitic (pea-sized round concretions), soft (roughly H 1–3), light for its size (SG about 2–2.5), with no fizz. It is the ore of aluminum and the main source of gallium as a by-product. Aluminum is the most abundant metal in the crust, but in common rocks like shale it is locked in silicate structures that are expensive to break; in bauxite it is already in hydroxides that dissolve in hot sodium hydroxide. Refining takes two steps: the Bayer process dissolves the aluminum hydroxides in hot NaOH and precipitates pure alumina (Al₂O₃), and then Hall–Héroult electrolysis splits alumina dissolved in molten cryolite into aluminum metal (and oxygen, which burns the carbon anodes to CO₂), using very large amounts of electricity.

Environments and primary structures

A depositional environment is the physical, chemical and biological setting where sediment accumulates: its energy, water depth, chemistry and organisms. Each leaves a recognizable combination of grain size, sorting, rounding, composition, structures and fossils. Thick piles of sedimentary rock (kilometers in sedimentary basins) can accumulate only where the crust slowly sinks, a process called subsidence (caused by stretching and cooling of the lithosphere, or by the weight of sediment, ice or mountain belts); subsidence makes room for each new layer and buries older ones deeper. The rules' list of environments is open, so learn the general logic as well as the table. Division C rules label the deep sea as abyssal.

ReferenceDepositional environments· 18 rows

Energy: H = high, M = moderate, L = low. Non-marine environments first, then transitional, then marine from shallow to deep.

18 rows

Environment	Energy	Typical sediment and rocks	Typical structures and features
Glacial	Variable (ice carries everything)	Till/tillite: unsorted mix of clay to boulders, angular striated clasts; outwash sand and gravel	No bedding in till; moraines; varves (annual light/dark couplets) in glacial lakes
Alluvial fan	H, sudden drop where a steep stream leaves a mountain front	Poorly sorted gravel and sand: breccia, conglomerate, arkose	Wedge-shaped deposits, debris-flow beds, coarsest near the mountain, red color in dry climates
Braided river	H	Gravel and coarse sand: conglomerate, sandstone	Many shifting channels and bars; cross-bedding
Meandering river (channel)	M	Sand, rounded gravel at the base of channels: sandstone, conglomerate	Cross-bedding, current (asymmetric) ripples, fining-upward channel fills, imbrication
Floodplain	L	Silt and mud from overbank floods: siltstone, shale, mudstone	Thin lamination, mud cracks, root traces, plant fossils, red soils
Lake (lacustrine)	L	Mud, silt, some carbonate or diatomite; evaporites in desert lakes	Fine lamination, varves, mud cracks at margins, fish and plant fossils
Swamp / bog	L, waterlogged, anoxic	Peat → coal; dark organic shale	Plant fossils, coal seams, root traces
Desert / aeolian (wind)	M (wind)	Very well sorted, well-rounded, frosted quartz sand: quartz sandstone; loess (silt) downwind	Large-scale cross-bedding (sets meters thick, from dunes), wind ripples, red color, no mud; Checkerboard Mesa and the Navajo Sandstone of Zion
Playa / salt flat (sabkha)	L	Evaporites: rock salt, rock gypsum; red mud	Mud cracks, salt crystal casts, lamination
Delta	M → L seaward	Sand at the delta front grading to silt and mud offshore; coal in delta-plain swamps	Cross-bedding, ripples, coarsening-upward sequences, plant fragments
Beach (shoreline)	H (waves)	Well-sorted, well-rounded quartz sand: quartz sandstone; shell beds: coquina	Gently dipping plane laminae, symmetric wave ripples, some cross-bedding, burrows
Tidal flat	L–M, alternating	Interlayered sand and mud; carbonate mud and dolomite in arid tropics	Mud cracks, ripples (directions reversing), stromatolites, burrows
Lagoon (behind barrier islands or reefs)	L, restricted	Mud, lime mud, sometimes evaporites	Fine lamination, burrows, restricted fauna
Reef	H on the front	Limestone built in place by corals, algae, sponges	Unbedded massive limestone, abundant fossils in growth position
Carbonate shelf / platform	L–M, warm shallow water	Fossil limestone, oolitic limestone, chalk farther out	Fossils, ooids, shell beds, burrows
Clastic continental shelf	M near shore → L offshore	Sand near shore grading to mud: sandstone, siltstone, shale	Symmetric and current ripples, storm beds, burrows, marine fossils
Continental slope and rise (submarine fans)	Episodic H (turbidity currents) between long L periods	Greywacke interbedded with shale (turbidites)	Graded bedding, repeated graded beds, sole marks on bed bases
Abyssal plain (deep ocean)	L	Fine clay, siliceous ooze (→ chert, diatomite) and, above the carbonate compensation depth, calcareous ooze (→ chalk); distal turbidites	Fine lamination, graded beds from distant turbidity currents

Where a mountain range rises, its weight bends the crust down in front of it into a foreland basin. Rapid erosion of the young mountains fills the basin with thick conglomerate and sandstone shed off the range, called molasse (named for the Molasse Basin north of the Alps); a molasse basin therefore records mountain building nearby.

Primary sedimentary structures

Primary structures form during or just after deposition, before lithification. They record how the sediment moved and which way was up. Secondary features form later, during diagenesis (foliation and slaty cleavage are metamorphic, not sedimentary):

Turbidites. The bed (and, once lithified, the rock) left by a single turbidity current is a turbidite. It has a sharp, often sole-marked base and is normally graded, so grain size increases from top to bottom. A complete turbidite shows the Bouma sequence of divisions, from base to top: massive or graded sand (A) → parallel-laminated sand (B) → ripple cross-laminated fine sand and silt (C) → laminated silt and mud (D) → mud that settled slowly afterwards (E). Turbidite successions on submarine fans and at the base of the continental slope stack hundreds of such sand–mud couplets; the typical turbidite sandstone is greywacke, its muddy matrix and poor sorting reflecting rapid dumping from a dense flow.

Metamorphic rocks

Metamorphic rocks form when an existing rock (the protolith, or parent rock) is changed by heat, pressure or chemically active fluids while it stays solid. Metamorphism begins at roughly 150–200 °C, above diagenesis, and ends when the rock starts to melt, at about 650–700 °C for water-saturated granite-like rock (higher for dry or mafic rock). Minerals recrystallize into new ones stable under the new conditions, grains grow, and directed pressure aligns them. Vesicles, flow banding and glass are igneous features; they do not form by metamorphism. Per-specimen data are in the rock reference tables.

Agents of metamorphism

Heat drives the reactions. It comes from burial (temperature rises with depth along the geothermal gradient, typically 25–30 °C per km in continental crust), from nearby magma, and from tectonic thickening of crust. Heat lets atoms diffuse through solid crystals and rearrange into new minerals without melting.

Pressure has two forms:

• Confining (lithostatic) pressure is equal in all directions, from the weight of overlying rock: about 1 kbar (100 MPa) per 3.5 km of depth (roughly 0.3 kbar per km). It makes minerals denser and more compact (kyanite instead of andalusite, garnet, omphacite in eclogite) but does not align them.
• Directed (differential) stress is greater in one direction, at plate boundaries where crust is squeezed. It flattens grains and makes platy and elongate minerals grow or rotate perpendicular to the maximum stress, producing foliation and lineation.

Chemically active fluids (hot water with dissolved CO₂, ions and silica) speed up reactions by carrying ions between grains, and can add or remove elements. When fluids significantly change the bulk chemical composition of the rock, the process is metasomatism.

Changes during metamorphism: recrystallization (existing minerals regrow larger, as calcite in marble), neocrystallization (new minerals not present in the protolith grow, as garnet in schist), phase change (a mineral turns into a polymorph, as andalusite to sillimanite), pressure solution, and plastic deformation. Porosity decreases: metamorphic rocks are denser and less porous than their protoliths. Low-grade metamorphic rocks can still contain recognizable (often stretched) fossils.

Brittle versus ductile. Cool, lightly confined rock near the surface is brittle: it cracks, faults and shatters into breccia. Hot, deeply buried rock under slow, long-lasting stress is ductile: it bends, folds and stretches without breaking, giving the contorted layers of schist and gneiss. The switch lies roughly 10–15 km down in continental crust, so shallow fault zones make fault breccia and deep ones make mylonite.

Types of metamorphism

Metasomatism and skarns. A hot pluton drives groundwater convection and releases its own fluids, which carry silica, iron and metals into the surrounding rock. When the country rock is limestone or dolostone, the reaction produces skarn, a coarse calc-silicate rock with garnet (grossular, andradite), diopside, wollastonite and epidote, often with ores of copper, tungsten, iron, zinc or gold. A simpler example is the reaction calcite + quartz → wollastonite + CO₂ in heated siliceous limestone, or dolomite + quartz + water → tremolite + calcite + CO₂ at low grade.

Serpentinization is hydrothermal metamorphism of peridotite: water added at low to moderate temperature converts olivine and pyroxene to serpentine minerals (plus magnetite and sometimes talc), producing green, waxy, often slickensided serpentinite. It happens where seawater penetrates the mantle at mid-ocean ridges and in subduction zones. Serpentinite and peridotite are exposed on land in ophiolites (see Ophiolites and the structure of oceanic lithosphere). Further hydration and CO₂ alteration of ultramafic rock gives talc schist and soapstone.

Texture: foliated or not

Foliation is any planar fabric in a metamorphic rock: parallel alignment of platy minerals (micas, chlorite) or segregation into layers. It forms under directed stress, perpendicular to the maximum squeeze, so it is a signature of regional metamorphism. It is not the same as sedimentary bedding: slaty cleavage often cuts across the original bedding at an angle.

The four foliated textures form a sequence of increasing grade and grain size:

Non-foliated (granoblastic or hornfelsic) rocks have interlocking, roughly equal-sized grains with no alignment. A rock is non-foliated when (1) its minerals are equidimensional (calcite in marble, quartz in quartzite), so there are no platy grains to line up even under directed stress, or (2) it formed without directed stress (contact metamorphism: hornfels; burial metamorphism). Equal confining pressure and heat alone give a non-foliated texture.

The five basic metamorphic textures are therefore slaty, phyllitic, schistose, gneissic and granoblastic (non-foliated). Other textures to recognize: porphyroblastic (large porphyroblasts in a finer matrix, the metamorphic counterpart of porphyritic), augen (eye-shaped feldspar porphyroclasts wrapped by foliation in augen gneiss) and lineation (aligned needles, as hornblende in amphibolite).

Why foliation wraps around porphyroblasts. Garnet and staurolite are strong, hard, roughly equant (blocky) crystals, while micas are weak, platy and easily bent. When the rock is flattened, the rigid porphyroblast resists and barely changes shape, so the weak micas flow and bend around it and the foliation curves around the crystal like water around a boulder, often with small "pressure shadows" of quartz on either side. Trails of tiny inclusions trapped inside a porphyroblast can preserve an older foliation, and S-shaped trails show that the crystal rotated as it grew during deformation.

Migmatite ("mixed rock") forms at the highest grades when part of a gneiss begins to melt: light, granitic melt (leucosome) collects in swirling veins and bands within the dark unmelted host (melanosome). It is part igneous and part metamorphic and marks the upper limit of metamorphism.

Protoliths

The protolith controls the chemistry; the conditions control the minerals and texture. Different protoliths under the same conditions give different rocks (shale → schist, limestone → marble, sandstone → quartzite side by side), and the same protolith at different grades gives a sequence (shale → slate → phyllite → schist → gneiss).

The rules' protolith pairs to know cold: shale → slate (and phyllite, schist, gneiss), limestone → marble, quartz sandstone → quartzite, peridotite → serpentinite (and serpentinite or ultramafic rock → soapstone), and basalt or gabbro → amphibolite. Gneiss can form from almost any protolith (granite, sandstone, shale, volcanic rocks). Marble cannot form from shale (no calcite), and quartzite cannot form from limestone. Graphite flakes in a schist or marble point to a protolith that held organic carbon, such as a dark, organic-rich shale or limestone.

Grade and index minerals

Metamorphic grade is the intensity of metamorphism, mainly the peak temperature reached (and the pressure): low grade (about 200–400 °C), medium (about 400–600 °C), high (about 600 °C and up). As grade increases, temperature and usually pressure increase, water is driven out of minerals (hydrous minerals like chlorite and micas give way to anhydrous ones like garnet, sillimanite and feldspar), grain size grows and foliation changes from slaty to gneissic. Grade is affected by temperature, pressure and the presence of fluids. Metamorphism on the way up in grade is prograde. Retrograde metamorphism happens as temperature and pressure fall again (during uplift and cooling): it needs water to get back into the rock, so it is usually patchy and incomplete, which is why high-grade minerals survive at the surface. Typical retrograde changes replace hornblende, biotite, garnet and pyroxene with lower-grade, water-bearing minerals such as chlorite (and epidote, actinolite or serpentine), turning an amphibolite greenish.

An index mineral forms only within a particular range of temperature and pressure in a rock of suitable composition, so its first appearance marks a grade. The 2027 rules close the index-mineral list at seven. Six of them are George Barrow's classic zones mapped in metamorphosed shale in the Scottish Highlands (1893–1912; the chlorite zone was formally added by Tilley in 1925): chlorite, biotite, garnet, staurolite, kyanite, sillimanite. Muscovite is added as the low-grade white mica that coarsens before biotite appears.

Temperatures are approximate (Barrovian pressures of roughly 4–8 kbar; they shift with pressure and rock composition). Index minerals appear in that order but earlier ones do not all vanish: a garnet-zone schist still has biotite and muscovite. At high grade, muscovite reacts with quartz to form sillimanite and K-feldspar (muscovite + quartz → sillimanite + K-feldspar + water), which is why the highest-grade pelitic gneisses lack muscovite.

An isograd is a line on a map joining the points where an index mineral first appears, a line of roughly equal grade. Crossing isograds toward the core of a mountain belt means rising temperature and pressure.

A sequence such as shale → slate → phyllite → schist → gneiss across a region (for example eastward from New York into New England) records grade increasing toward the deeper, hotter core of the old mountain belt; beyond the highest grade, the rock would melt and become igneous.

The Al₂SiO₅ polymorphs. Kyanite, andalusite and sillimanite have the same formula, so the one present depends only on pressure and temperature: andalusite at low pressure (contact aureoles, shallow crust), kyanite at high pressure and moderate temperature (it is the densest), and sillimanite at high temperature, on the hot side of both others. On a P–T diagram sillimanite's field lies to the right (higher temperature) of the andalusite field at low pressure and of the kyanite field at high pressure, so heating either one far enough converts it to sillimanite; that is why sillimanite marks the highest grade in both Barrovian (kyanite → sillimanite) and Buchan (andalusite → sillimanite) sequences. A rock whose aluminosilicates go kyanite → andalusite → sillimanite along its prograde path was heated while pressure fell (decompression during uplift). The three fields meet at a triple point near 500 °C and about 4 kbar (roughly 13–15 km depth). A kyanite-bearing rock was therefore buried deeper than an andalusite-bearing rock. In the north-east Scottish Highlands (Buchan type), andalusite replaces kyanite in the sequence because metamorphism happened at lower pressure. Kyanite also shows anisotropic hardness: about 4.5–5 along the length of its blades and 6.5–7 across them.

Corundum, epidote and andalusite are named in the rules as metamorphic minerals but not index minerals. Epidote is typical of greenschist-facies mafic rocks, so it still indicates low-to-medium-grade metamorphism. Talc, serpentine and actinolite–tremolite indicate low-grade metamorphism of ultramafic and dolomitic rocks.

Regional and contact metamorphism

A contact aureole is the zone of altered country rock around an intrusion. Grade is highest at the contact and decreases outward. A dike or sill bakes only a few centimeters to meters; a large batholith can produce an aureole kilometers wide. Shale becomes hornfels (hard, dense, fine-grained, non-foliated, often with spots of andalusite or cordierite), limestone becomes marble or skarn, sandstone becomes quartzite. Contact effects also prove in relative dating that the intrusion is younger than the rock it baked.

Metamorphic facies (Division C)

A metamorphic facies is a set of mineral assemblages that form together under a given range of pressure and temperature, whatever the protolith. Facies are named after the rock that a mafic (basaltic) protolith becomes under those conditions. Read them from a pressure–temperature graph: temperature on the horizontal axis, pressure (and depth) on the vertical axis, with pressure usually increasing upward in metamorphic diagrams (some diagrams plot depth increasing downward; check the axis).

Geotherms tie it together. The temperature gained per kilometer of depth decides which facies a rock passes through:

• Steep geotherm (high T at low P, near intrusions or over hot spots): hornfels facies → contact metamorphism (Buchan-type regional metamorphism is also relatively high T, low P).
• Moderate geotherm (about 25–30 °C/km, continental collision): zeolite → greenschist → amphibolite → granulite (Barrovian path).
• Low geotherm (about 10 °C/km or less, subducting slab): zeolite → blueschist → eclogite. The cold oceanic slab descends faster than it can heat up, so it reaches high pressure while still cool.

Plate-tectonic settings of metamorphism (Division C)

Gneiss and granulite make up much of the lower continental crust and of ancient continental shields; the "basement" under sedimentary cover in most continents is old igneous and high-grade metamorphic rock.

The rock cycle and uses of rocks and minerals

The rock cycle

The rock cycle links the three rock families through the processes that convert one into another. It runs on two energy sources: Earth's internal heat (from radioactive decay and heat left from Earth's formation), which drives plate motion, melting, metamorphism and uplift; and the Sun, which drives the water cycle, weather, and therefore weathering, erosion and deposition, together with gravity. Every arrow is a process, and the event tests the processes more than the labels.

Key points the questions probe:

• Any rock can become any other kind. A metamorphic rock becomes sedimentary by uplift, weathering, erosion, deposition and lithification; a sedimentary rock becomes igneous by burial, melting and crystallization; igneous rock becomes metamorphic if it stays buried and is heated and squeezed (granite → gneiss).
• Shortcuts are common: igneous rock can be metamorphosed without ever being sediment; metamorphic rock need not melt. "Metamorphic rocks always eventually melt" is false.
• Sedimentary rocks form only at or near the surface; metamorphic and plutonic igneous rocks form at depth and reach the surface only by uplift and erosion of the overlying rock.
• Grains survive between cycles. A quartz crystal from a granite can become a sand grain in a sandstone and then part of a quartzite; a zircon can survive several cycles, which is why detrital zircons older than their host rock are found.
• Matching questions: breccia → pegmatite needs melting then cooling; andesite → dolostone needs weathering and erosion (then deposition, precipitation and lithification); magma → gneiss needs crystallization (to granite) then regional metamorphism; granite → sandstone needs weathering, erosion, deposition, compaction and cementation.
• Human activity speeds up the surface part of the cycle: mining, construction, farming and deforestation expose and erode rock and soil far faster than natural rates, while dams trap sediment that rivers would carry to the sea.

Economic uses of rocks

The rules ask for the economic uses of rocks (uses of minerals, ores of individual minerals and gem varieties are covered in the minerals chapters; see How minerals form and the mineral reference tables). A rock's use follows from its properties: strength, hardness, how it splits, how it polishes, how easily it dissolves, porosity, and chemical composition.

Dating rocks and minerals

Geologists date rocks in two ways. Relative dating puts events in order (older or younger) using the geometry of rocks. Absolute (numerical, radiometric) dating gives an age in years from the decay of radioactive isotopes. Questions range from naming a principle shown in a sketch to computing an age from parent and daughter amounts.

Relative dating principles

Bedding planes and unconformities. A short pause in deposition makes a bedding plane; a long interval of erosion or non-deposition makes an unconformity, a gap in the rock record.

Quick sketches (younger rock on top, contact in the middle):

Marker beds. A volcanic ash layer (later tuff) falls over a huge area in days, so it is the same age everywhere: it correlates sequences between regions, and its crystals (zircon, sanidine, biotite) can be dated radiometrically to give the sediments above and below an absolute bracket.

Radioactive decay

Radioactive isotopes have unstable nuclei that change spontaneously into other nuclei. The original isotope is the parent; the product is the daughter. Some parents reach a stable daughter in one step (K-40 → Ar-40); others pass through a chain of short-lived intermediates (U-238 → … → Pb-206 by 8 alpha and 6 beta decays).

The key property: decay rate is fixed for each isotope and is not changed by temperature, pressure or chemical bonding, so it works as a clock. Each nucleus has a constant probability of decaying per unit time, the decay constant λ. That makes the decay exponential: in each half-life, half of the remaining parent decays.

(This table assumes no daughter at the start and that every decay produces one daughter atom that stays in the mineral.)

Isotope systems

Choosing a system. A system works when the age is roughly between 1/1000 of a half-life and a few half-lives: young enough that measurable parent remains, old enough that measurable daughter has formed. Rb–Sr cannot date a 10,000-year-old sample because only about one atom in 7 million would have decayed. C-14 cannot date a 300-million-year-old rock because none would remain (and rocks did not take up atmospheric carbon). Uranium–lead and potassium–argon (or argon–argon) are the standard choices for igneous rocks hundreds of millions of years old.

What can be dated. Radiometric ages date the moment a mineral crystallized or last cooled through its closure temperature. Igneous rocks are ideal: their minerals start with parent and no daughter. Metamorphic rocks can be dated too (the age of metamorphism, from minerals that grew or were reset). Sedimentary rocks are hard to date directly, because their grains are older detritus; a detrital zircon gives the age of the granite it eroded from, not of the sandstone. Sediments are dated indirectly with interbedded ash layers and cross-cutting intrusions.

Why zircon is ideal

• When zircon (ZrSiO₄) crystallizes, U⁴⁺ substitutes for Zr⁴⁺ (same charge, similar ionic radius), but Pb²⁺ (wrong charge, too large) is excluded. So virtually all the lead in a zircon is radiogenic: the starting daughter is effectively zero.
• It contains two uranium clocks (U-238 → Pb-206 and U-235 → Pb-207) with different half-lives. If both give the same age (concordant), the system was closed; if not, lead loss can be detected and corrected.
• It has a very high closure temperature (above about 900 °C), so later heating rarely resets it.
• It is hard (H 7.5), chemically resistant and survives weathering, erosion, transport and even metamorphism; it is common as tiny crystals in granites, rhyolites and tuffs. The oldest known terrestrial material is detrital zircon from the Jack Hills of Western Australia, about 4.4 billion years old, while the oldest known rock (Acasta Gneiss, Canada) is about 4.0 billion years old.

Why the oldest rocks do not give Earth's age. Earth's earliest crust has been destroyed by plate tectonics, melting, metamorphism and erosion, so the oldest surviving rock is younger than the planet. Earth's age (about 4.54–4.57 billion years) comes from meteorites and moon rocks, which formed with the solar system and have not been recycled.

Closure temperature

A mineral behaves as a closed system only when it is cool enough that daughter atoms cannot diffuse out. The closure temperature is the temperature below which a mineral effectively retains its daughter isotope. Above it, daughters escape as fast as they form and the clock stays at zero; once the mineral cools below it, the clock starts. If a rock is later reheated above the closure temperature (by metamorphism or an intrusion), the daughter is driven out and the clock resets. A K–Ar mineral that is completely outgassed and then cools gives an apparent age of zero at that moment, which then records the time since reheating.

Different minerals in one rock can therefore give different ages: zircon records crystallization of a granite, and biotite records when it later cooled to about 300 °C. Together they give a cooling history (thermochronology).

Problems with K–Ar dating: argon is a noble gas that leaks out of minerals that are reheated or altered, giving ages that are too young; excess argon trapped from the surroundings gives ages that are too old; potassium and argon must be measured on different portions of the sample; and only about 11% of K-40 decays to Ar-40, so the branching ratio must be included. The Ar–Ar method irradiates the sample to turn K-39 into Ar-39, so both are measured as argon isotopes in one step-heating experiment that also reveals argon loss.

Isochron dating (enrichment) measures several minerals or samples from the same rock that started with the same Sr-87/Sr-86 ratio but different amounts of Rb. On a plot of Sr-87/Sr-86 against Rb-87/Sr-86 they fall on a straight line (an isochron) whose slope gives the age and whose intercept gives the initial ratio. It removes the need to assume zero initial daughter, the main unknown in simple dating.

Age calculations

Three quantities are needed for an absolute age: the amount of parent remaining, the amount of daughter formed, and the half-life. The amount of parent the rock started with is not measured directly; it is inferred as P + D.

Thin sections (Division C, State and National)

The petrographic microscope: PPL and XPL

Light from the lamp passes through a polarizer (lower polar) so that it vibrates in a single plane: plane-polarized light (PPL). Above the sample sits a second polarizer, the analyzer, oriented at 90° to the first. With the analyzer inserted you see cross-polarized light (XPL, "crossed nicols"): with no sample, or with an isotropic sample, the field is black, because the analyzer blocks all light vibrating in the polarizer's plane.

An anisotropic mineral (any non-cubic crystal) splits light into two rays vibrating at right angles, traveling at different speeds with different refractive indices (double refraction; see calcite's double image in hand specimen). When they recombine at the analyzer they interfere, producing interference colors. Isotropic substances (cubic minerals such as garnet, halite, fluorite, diamond; and glass) have one refractive index in all directions, do not split light, and stay black in XPL.

Dichroism versus trichroism. How many pleochroic colors a mineral can show depends on its optical class. Uniaxial minerals (tetragonal, hexagonal, trigonal) have two principal refractive indices, so they show at most two colors (dichroic): tourmaline changes between pale and dark brown or green as the stage turns. Biaxial minerals (orthorhombic, monoclinic, triclinic) have three principal indices and can show three colors (trichroic): cordierite (gem iolite: violet-blue, pale blue, near colorless), tanzanite (blue, violet, burgundy) and orthopyroxene (hypersthene: pink, yellow, green). Some biaxial minerals look two-colored because two of their colors are nearly identical (biotite: brown and pale tan). Isotropic minerals (cubic, glass) show one color only.

Optical properties

Birefringence (δ) is the difference between a mineral's highest and lowest refractive indices. The interference color depends on the retardation (Δ, the path difference between the two rays), which is birefringence times thickness. Colors are read on the Michel-Lévy chart in orders: first order (0 to about 550 nm: black → gray → white → yellow → red), second order (about 550–1100 nm: blue, green, yellow, red, vivid), third order (about 1100–1650 nm: paler green, pink), and above that pale, creamy "high-order white".

• The maximum interference color of a mineral is seen in grains cut parallel to the optic axis (the direction showing both extreme refractive indices). A grain cut perpendicular to the optic axis looks isotropic (dark) because light traveling along the optic axis is not split. Use the highest color seen among many grains.
• Extinction happens every 90° of rotation. Parallel extinction: the grain goes dark when its cleavage or length is parallel to a crosshair (micas, orthopyroxene). Inclined extinction: it goes dark at an angle to the cleavage, the extinction angle (augite about 35–48°; plagioclase angle depends on its An content, which is how petrographers estimate plagioclase composition). Undulose (wavy) extinction: a dark band sweeps across a strained grain as you rotate (common in quartz). Symmetrical extinction bisects two cleavages (amphibole basal sections).
• Relief depends on the difference between the mineral's refractive index and that of the epoxy (≈1.54). Diamond's refractive index is very high (≈2.42), but rutile is higher still in its extraordinary ray (≈2.9), so among rutile, diamond, calcite and zircon, rutile is highest. A higher refractive index means a lower critical angle for total internal reflection.

Five minerals in thin section

Other minerals that appear in photomicrographs as distractors: quartz (colorless, low relief, no cleavage, first-order gray/white, undulose extinction, rarely altered and with no visible twinning in thin section); olivine (high relief, no good cleavage, irregular fractures, bright second- to third-order colors, often altering along cracks); biotite (strong brown pleochroism, one cleavage, parallel "bird's-eye" speckled extinction); muscovite (colorless, one cleavage, bright second- to third-order colors); hornblende (green-brown pleochroic, 56°/124° cleavage).

Igneous textures under the microscope

Sedimentary textures under the microscope

Clastic sedimentary rocks show discrete grains held by cement (calcite, silica, iron oxide) or a fine matrix, with pore space (often dyed blue) between them; igneous rocks show interlocking crystals with no pores. Read four things:

Specimen checklist (A–Z)

Every specimen on the National List in alphabetical order, with its family, whether it is State and National only (*), whether the supervisor supplies identifying data (¹), and where to find its property line and photos. Print this page as the index of your binder.

ReferenceNational List checklist· 129 rows

Type a name, a family, * or ¹ in the filter box.

129 rows

Specimen	Rock or mineral	Family on the list	*	¹	Property table	Photos
Actinolite	Mineral	Silicates	*		Silicates: amphiboles to kyanite	Photos: silicates, amphiboles to kyanite
Agate/Chalcedony	Mineral	Silicates			Quartz varieties	Photos: quartz varieties and opal
Albite	Mineral	Silicates			Silicates: amphiboles to kyanite	Photos: silicates, amphiboles to kyanite
Almandine	Mineral	Silicates			Silicates: amphiboles to kyanite	Photos: silicates, amphiboles to kyanite
Amazonite	Mineral	Silicates			Silicates: amphiboles to kyanite	Photos: silicates, amphiboles to kyanite
Amethyst	Mineral	Silicates			Quartz varieties	Photos: quartz varieties and opal
Amphibolite	Rock	Metamorphic	*		Metamorphic rock table	Photos: metamorphic rocks
Andesite	Rock	Igneous			Igneous rock table	Photos: igneous rocks
Anthracite	Rock	Sedimentary			Sedimentary rock table	Photos: sedimentary rocks
Apophyllite	Mineral	Silicates	*		Silicates: amphiboles to kyanite	Photos: silicates, amphiboles to kyanite
Aragonite	Mineral	Carbonates			Carbonates and borates	Photos: carbonates and borates
Arkose	Rock	Sedimentary			Sedimentary rock table	Photos: sedimentary rocks
Augite	Mineral	Silicates			Silicates: micas to zircon	Photos: silicates, micas to zircon
Aventurine	Mineral	Silicates			Quartz varieties	Photos: quartz varieties and opal
Azurite	Mineral	Carbonates			Carbonates and borates	Photos: carbonates and borates
Banded Iron Formation	Rock	Sedimentary			Sedimentary rock table	Photos: sedimentary rocks
Barite	Mineral	Sulfates			Sulfates	Photos: sulfates
Basalt	Rock	Igneous			Igneous rock table	Photos: igneous rocks
Bauxite	Rock	Sedimentary			Sedimentary rock table	Photos: sedimentary rocks
Beryl	Mineral	Silicates			Silicates: amphiboles to kyanite	Photos: silicates, amphiboles to kyanite
Biotite	Mineral	Silicates			Silicates: micas to zircon	Photos: silicates, micas to zircon
Bituminous	Rock	Sedimentary			Sedimentary rock table	Photos: sedimentary rocks
Bornite	Mineral	Sulfides	*	¹	Sulfides	Photos: sulfides
Breccia	Rock	Sedimentary			Igneous rock table	Photos: igneous rocks
Calcite	Mineral	Carbonates			Carbonates and borates	Photos: carbonates and borates
Celestite	Mineral	Sulfates			Sulfates	Photos: sulfates
Chalcopyrite	Mineral	Sulfides			Sulfides	Photos: sulfides
Chalk	Rock	Sedimentary			Sedimentary rock table	Photos: sedimentary rocks
Chert/Flint	Rock	Sedimentary			Sedimentary rock table	Photos: sedimentary rocks
Chrysocolla	Mineral	Silicates	*		Silicates: amphiboles to kyanite	Photos: silicates, amphiboles to kyanite
Citrine	Mineral	Silicates			Quartz varieties	Photos: quartz varieties and opal
Conglomerate	Rock	Sedimentary			Sedimentary rock table	Photos: sedimentary rocks
Copper	Mineral	Native Elements			Native elements	Photos: native elements
Coquina	Rock	Sedimentary			Sedimentary rock table	Photos: sedimentary rocks
Corundum	Mineral	Oxide/Hydroxides			Oxides and hydroxides	Photos: oxides and hydroxides
Diamond	Mineral	Native Elements			Native elements	Photos: native elements
Diatomite	Rock	Sedimentary	*	¹	Sedimentary rock table	Photos: sedimentary rocks
Dioptase	Mineral	Silicates	*		Silicates: amphiboles to kyanite	Photos: silicates, amphiboles to kyanite
Diorite	Rock	Igneous			Igneous rock table	Photos: igneous rocks
Dolomite	Mineral	Carbonates			Carbonates and borates	Photos: carbonates and borates
Dolostone	Rock	Sedimentary			Sedimentary rock table	Photos: sedimentary rocks
Epidote	Mineral	Silicates			Silicates: amphiboles to kyanite	Photos: silicates, amphiboles to kyanite
Fluorapatite	Mineral	Phosphates			Phosphates	Photos: phosphates
Fluorite	Mineral	Halides			Halides	Photos: halides
Fossil Limestone	Rock	Sedimentary			Sedimentary rock table	Photos: sedimentary rocks
Gabbro	Rock	Igneous			Igneous rock table	Photos: igneous rocks
Galena	Mineral	Sulfides			Sulfides	Photos: sulfides
Garnet Schist	Rock	Metamorphic			Metamorphic rock table	Photos: metamorphic rocks
Gneiss	Rock	Metamorphic			Metamorphic rock table	Photos: metamorphic rocks
Goethite/Limonite	Mineral	Oxide/Hydroxides			Oxides and hydroxides	Photos: oxides and hydroxides
Gold	Mineral	Native Elements			Native elements	Photos: native elements
Granite	Rock	Igneous			Igneous rock table	Photos: igneous rocks
Graphite	Mineral	Native Elements			Native elements	Photos: native elements
Greywacke	Rock	Sedimentary	*		Sedimentary rock table	Photos: sedimentary rocks
Grossular (green)	Mineral	Silicates			Silicates: amphiboles to kyanite	Photos: silicates, amphiboles to kyanite
Gypsum: Alabaster (massive)	Mineral	Sulfates			Sulfates	Photos: sulfates
Gypsum: Satin Spar (fibrous)	Mineral	Sulfates			Sulfates	Photos: sulfates
Gypsum: Selenite (crystalline)	Mineral	Sulfates			Sulfates	Photos: sulfates
Halite	Mineral	Halides			Halides	Photos: halides
Hematite	Mineral	Oxide/Hydroxides			Oxides and hydroxides	Photos: oxides and hydroxides
Hornblende	Mineral	Silicates			Silicates: amphiboles to kyanite	Photos: silicates, amphiboles to kyanite
Jasper	Mineral	Silicates	*		Quartz varieties	Photos: quartz varieties and opal
Kaolinite	Mineral	Silicates			Silicates: amphiboles to kyanite	Photos: silicates, amphiboles to kyanite
Kyanite	Mineral	Silicates			Silicates: amphiboles to kyanite	Photos: silicates, amphiboles to kyanite
Labradorite	Mineral	Silicates			Silicates: amphiboles to kyanite	Photos: silicates, amphiboles to kyanite
Lepidolite	Mineral	Silicates	*		Silicates: micas to zircon	Photos: silicates, micas to zircon
Lignite	Rock	Sedimentary			Sedimentary rock table	Photos: sedimentary rocks
Magnesite	Mineral	Carbonates	*	¹	Carbonates and borates	Photos: carbonates and borates
Magnetite	Mineral	Oxide/Hydroxides			Oxides and hydroxides	Photos: oxides and hydroxides
Malachite	Mineral	Carbonates			Carbonates and borates	Photos: carbonates and borates
Marble	Rock	Metamorphic			Metamorphic rock table	Photos: metamorphic rocks
Mica Schist	Rock	Metamorphic			Metamorphic rock table	Photos: metamorphic rocks
Milky Quartz	Mineral	Silicates			Quartz varieties	Photos: quartz varieties and opal
Muscovite	Mineral	Silicates			Silicates: micas to zircon	Photos: silicates, micas to zircon
Obsidian	Rock	Igneous			Igneous rock table	Photos: igneous rocks
Olivine	Mineral	Silicates			Silicates: micas to zircon	Photos: silicates, micas to zircon
Oolitic Limestone	Rock	Sedimentary			Sedimentary rock table	Photos: sedimentary rocks
Opal	Mineral	Silicates			Quartz varieties	Photos: quartz varieties and opal
Orthoclase/Microcline	Mineral	Silicates			Silicates: amphiboles to kyanite	Photos: silicates, amphiboles to kyanite
Pegmatite	Rock	Igneous			Igneous rock table	Photos: igneous rocks
Peridotite	Rock	Igneous			Igneous rock table	Photos: igneous rocks
Phyllite	Rock	Metamorphic			Metamorphic rock table	Photos: metamorphic rocks
Prehnite	Mineral	Silicates	*		Silicates: micas to zircon	Photos: silicates, micas to zircon
Pumice	Rock	Igneous			Igneous rock table	Photos: igneous rocks
Pyrite	Mineral	Sulfides			Sulfides	Photos: sulfides
Pyrolusite	Mineral	Oxide/Hydroxides	*		Oxides and hydroxides	Photos: oxides and hydroxides
Pyromorphite	Mineral	Phosphates	*		Phosphates	Photos: phosphates
Quartz Sandstone	Rock	Sedimentary			Sedimentary rock table	Photos: sedimentary rocks
Quartzite	Rock	Metamorphic			Metamorphic rock table	Photos: metamorphic rocks
Rhodochrosite	Mineral	Carbonates	*		Carbonates and borates	Photos: carbonates and borates
Rhodonite	Mineral	Silicates	*		Silicates: micas to zircon	Photos: silicates, micas to zircon
Rhyolite	Rock	Igneous			Igneous rock table	Photos: igneous rocks
Rock Crystal	Mineral	Silicates			Quartz varieties	Photos: quartz varieties and opal
Rock Gypsum	Rock	Sedimentary			Sedimentary rock table	Photos: sedimentary rocks
Rock Salt (Halite)	Rock	Sedimentary			Sedimentary rock table	Photos: sedimentary rocks
Rose Quartz	Mineral	Silicates			Quartz varieties	Photos: quartz varieties and opal
Rutile	Mineral	Oxide/Hydroxides	*		Oxides and hydroxides	Photos: oxides and hydroxides
Scoria	Rock	Igneous			Igneous rock table	Photos: igneous rocks
Serpentinite	Rock	Metamorphic	*		Metamorphic rock table	Photos: metamorphic rocks
Shale	Rock	Sedimentary			Sedimentary rock table	Photos: sedimentary rocks
Siderite	Mineral	Carbonates	*		Carbonates and borates	Photos: carbonates and borates
Siltstone	Rock	Sedimentary	*	¹	Sedimentary rock table	Photos: sedimentary rocks
Silver	Mineral	Native Elements			Native elements	Photos: native elements
Slate	Rock	Metamorphic			Metamorphic rock table	Photos: metamorphic rocks
Smoky Quartz	Mineral	Silicates	*		Quartz varieties	Photos: quartz varieties and opal
Sodalite	Mineral	Silicates			Silicates: micas to zircon	Photos: silicates, micas to zircon
Sphalerite	Mineral	Sulfides			Sulfides	Photos: sulfides
Spodumene	Mineral	Silicates	*		Silicates: micas to zircon	Photos: silicates, micas to zircon
Staurolite	Mineral	Silicates			Silicates: micas to zircon	Photos: silicates, micas to zircon
Stibnite	Mineral	Sulfides	*		Sulfides	Photos: sulfides
Stilbite	Mineral	Silicates	*		Silicates: micas to zircon	Photos: silicates, amphiboles to kyanite
Sulfur	Mineral	Native Elements			Native elements	Photos: native elements
Syenite	Rock	Igneous	*	¹	Igneous rock table	Photos: igneous rocks
Talc	Mineral	Silicates			Silicates: micas to zircon	Photos: silicates, micas to zircon
Talc Schist (Soapstone)	Rock	Metamorphic	*		Metamorphic rock table	Photos: metamorphic rocks
Tiger’s Eye	Mineral	Silicates	*		Quartz varieties	Photos: quartz varieties and opal
Topaz	Mineral	Silicates			Silicates: micas to zircon	Photos: silicates, micas to zircon
Tourmaline	Mineral	Silicates			Silicates: micas to zircon	Photos: silicates, micas to zircon
Travertine	Rock	Sedimentary			Sedimentary rock table	Photos: sedimentary rocks
Tremolite	Mineral	Silicates	*		Silicates: amphiboles to kyanite	Photos: silicates, amphiboles to kyanite
Tuff	Rock	Igneous	*	¹	Igneous rock table	Photos: igneous rocks
Tuff Breccia	Rock	Igneous	*		Igneous rock table	Photos: igneous rocks
Turquoise	Mineral	Phosphates	*		Phosphates	Photos: phosphates
Ulexite	Mineral	Borates			Carbonates and borates	Photos: carbonates and borates
Vanadinite	Mineral	Phosphates	*		Phosphates	Photos: phosphates
Willemite	Mineral	Silicates	*		Silicates: micas to zircon	Photos: silicates, micas to zircon
Wollastonite	Mineral	Silicates	*		Silicates: micas to zircon	Photos: silicates, micas to zircon
Zincite	Mineral	Oxide/Hydroxides	*		Oxides and hydroxides	Photos: oxides and hydroxides
Zircon	Mineral	Silicates	*		Silicates: micas to zircon	Photos: silicates, micas to zircon

Mineral reference tables

These tables give one property line for every mineral on the National List, one table per chemical family: first the non-silicates in the order the list prints them, then the silicates, the quartz varieties, the rules-named minerals that are not on the ID list, the gem varieties and a single table of mineral look-alike pairs. At an identification station, start with luster: if the specimen is metallic, go straight to the "Metallic minerals at a glance" table (after the phosphates table), which is sorted by streak, the fastest way to narrow a metallic sample. For a nonmetallic sample, use hardness, cleavage and the special tests in the "Habit & diagnostic tests" column (acid, magnetism, fluorescence, heft) to pick between the few candidates left. Many written questions ask for a formula, an ore, a use or a locality, so read the last two columns as well as the physical properties. * = State/National only; ¹ = the supervisor supplies diagnostic information (see Supplied-information specimens (¹)).

Native elements

ReferenceNative elements: reference table· 6 rows

H = Mohs hardness; SG = specific gravity. Native metals are all isometric, heavy, malleable and lack cleavage. Gold, silver and copper share one structure (gold group): heavy atoms in cubic closest packing (face-centered cubic), which makes them dense and malleable.

6 rows

Mineral	Formula	Class & group	Crystal system	H	SG	Luster	Color	Streak	Cleavage / fracture	Habit & diagnostic tests	Forms in & notable localities	Uses, ores & gem varieties
Copper	Cu	Native element; metal (gold group)	Isometric	2.5–3	8.9	Metallic	Copper-red on fresh surface; tarnishes brown to black; green patina on weathering (mostly basic copper sulfates and carbonates)	Copper-red, shiny	none; hackly fracture	Dendritic, wiry, arborescent, sheets and nuggets, rare cubes/dodecahedra; malleable and ductile; dissolves in nitric acid (blue-green solution, brown fumes); no fizz in HCl; not magnetic	Oxidized zones of copper deposits; cavities and amygdules in basalt flows. Keweenaw Peninsula, Michigan; Ray and Bisbee, Arizona	Wire, electrical and plumbing; alloys brass (Cu–Zn) and bronze (Cu–Sn); minor ore of copper; name from Latin cuprum, "metal of Cyprus"
Diamond	C	Native element; nonmetal (carbon polymorph)	Isometric	10	3.52	Adamantine; greasy on rough crystals	Colorless, yellow, brown; rarely blue, pink, green	none (harder than streak plate)	Perfect octahedral, 4 directions; conchoidal fracture	Octahedra, often with curved faces and triangular pits (trigons); brittle despite H 10; very high refractive index (2.42) and dispersion ("fire"); many fluoresce blue under LW UV; excellent heat conductor; grease-loving (sticks to grease tables)	Crystallizes in the mantle (>150 km) and is carried up in kimberlite and lamproite pipes; also placers. Kimberley, South Africa; Jwaneng, Botswana; Mir pipe, Siberia. Most natural diamonds are 1 to 3.5 billion years old; nitrogen is their commonest impurity (yellow tint)	Gemstone; industrial abrasive, saw blades, drill bits, polishing powder; polymorph of graphite; name from Greek adamas, "unconquerable"
Gold	Au	Native element; metal (gold group)	Isometric	2.5–3	19.3 (15–19.3 with Ag)	Metallic	Rich golden yellow, paler with silver; never tarnishes	Golden yellow, shiny	none; hackly fracture	Nuggets, flakes, grains, wires, dendrites, rare octahedra; very malleable, ductile and sectile (cuts with a knife); extremely heavy; insoluble in single acids but dissolves in aqua regia; not magnetic; unlike pyrite and chalcopyrite it dents instead of crumbling	Hydrothermal quartz veins; placers (streams); ancient placer conglomerates. Witwatersrand, South Africa; Mother Lode, California; Kalgoorlie, Western Australia	Monetary reserve, jewellery, electronics (corrosion-proof contacts), dentistry; ore of gold (much is "invisible" gold locked in pyrite and arsenopyrite, freed by roasting or oxidation); electrum = natural Au–Ag alloy (paler)
Graphite	C	Native element; nonmetal (carbon polymorph)	Hexagonal	1–2	2.1–2.3	Metallic to submetallic; dull when earthy	Black to steel-gray	Black to dark gray, shiny	Perfect basal, 1 direction; thin flakes flexible but not elastic	Foliated, scaly or earthy masses; rare hexagonal plates; greasy feel; marks paper and soils fingers; conducts electricity; very light for a metallic-looking mineral	Metamorphism of organic-rich sediments (graphitic schist, gneiss, marble); metamorphosed coal; veins. Borrowdale, England; Sri Lanka (vein graphite)	Pencil "lead" (with clay), dry lubricant, lithium-ion battery anodes, electrodes, crucibles and refractories, nuclear reactor moderator; name from Greek graphein, "to write"
Silver	Ag	Native element; metal (gold group)	Isometric	2.5–3	10.5 (10.1–11.1)	Metallic	Silver-white; tarnishes gray to black	Silver-white, shiny	none; hackly fracture	Wires (often curled), dendrites, plates, scales; malleable and ductile; dissolves in nitric acid; black tarnish is diagnostic against platinum-like metals	Hydrothermal veins and the oxidized/enriched zone of silver deposits. Kongsberg, Norway (wire silver); Cobalt, Ontario; Keweenaw, Michigan (with copper)	Jewellery and silverware, electronics, solar cells, mirrors, photography (historic); ore of silver (most silver is a by-product of galena and copper ores)
Sulfur	S	Native element; nonmetal	Orthorhombic	1.5–2.5	2.0–2.1	Resinous to greasy; adamantine on crystal faces	Bright yellow; brownish or greenish with impurities	White to pale yellow	Poor; conchoidal to uneven fracture	Dipyramidal crystals, crusts, earthy masses; very brittle; low melting point (about 115 °C), burns with blue flame and choking SO₂ smell; poor heat conductor (crystals may crack from the warmth of a hand); insoluble in water	Volcanic fumaroles and crater deposits; caprock of salt domes (bacteria reduce gypsum/anhydrite); sedimentary beds. Agrigento, Sicily; Gulf Coast salt domes, Louisiana–Texas; Kawah Ijen, Indonesia	Sulfuric acid (main use), fertilizers, vulcanizing rubber, gunpowder, fungicides and matches; old name "brimstone" ("burning stone")

Sulfides

ReferenceSulfides: reference table· 6 rows

* = State/National only; ¹ = supervisor supplies diagnostic information. All are brittle metallic minerals except sphalerite (usually resinous).

6 rows

Mineral	Formula	Class & group	Crystal system	H	SG	Luster	Color	Streak	Cleavage / fracture	Habit & diagnostic tests	Forms in & notable localities	Uses, ores & gem varieties
Bornite*¹	Cu₅FeS₄	Sulfide	Orthorhombic (pseudo-cubic; isometric above roughly 200–265 °C)	3–3.25	5.06–5.09	Metallic	Coppery red to bronze-brown on fresh surface; quickly tarnishes iridescent purple-blue ("peacock ore")	Grayish black	Poor to none; uneven to conchoidal fracture	Usually massive; rare rough cubes; iridescent purple tarnish is the key clue (but acid-treated chalcopyrite sold as "peacock ore" looks similar; check fresh color: bornite is coppery, chalcopyrite brassy)	Porphyry copper deposits, hydrothermal veins, supergene enrichment zones, mafic intrusions. Butte, Montana; Bristol, Connecticut; Dzhezkazgan, Kazakhstan	Important ore of copper; named after Austrian mineralogist Ignaz von Born
Chalcopyrite	CuFeS₂	Sulfide	Tetragonal	3.5–4	4.1–4.3	Metallic	Brass-yellow, often with a greenish tint; iridescent tarnish	Greenish black	Poor; uneven fracture	Usually massive; crystals look like tetrahedra (disphenoids); brittle; a steel knife scratches it (pyrite is too hard); deeper yellow than pyrite	Hydrothermal veins, porphyry copper deposits, volcanogenic massive sulfides, magmatic Ni–Cu sulfides. Bingham Canyon, Utah; Chuquicamata, Chile; Cornwall, England	Most important ore of copper; name from Greek chalkos ("copper") + pyrite
Galena	PbS	Sulfide (galena group)	Isometric	2.5–2.75	7.4–7.6	Metallic (bright on fresh cleavage, dull gray when tarnished)	Lead-gray	Lead-gray	Perfect cubic, 3 directions at 90°; breaks into small cubes and stair-step faces	Cubes and cubo-octahedra, but most often massive or coarsely granular cleavable masses; very heavy; brittle; warm HCl slowly releases H₂S (rotten-egg smell); not magnetic	Hydrothermal veins and carbonate-hosted (Mississippi Valley-type) deposits. Tri-State district (Missouri–Kansas–Oklahoma); Viburnum Trend, Missouri; Broken Hill, Australia	Main ore of lead (batteries, radiation shielding); by-product silver (argentiferous galena); early crystal-radio detectors; ancient kohl eye make-up (lead-based kohl causes lead poisoning); name from Latin galena, "lead ore"
Pyrite	FeS₂	Sulfide (pyrite group)	Isometric	6–6.5	4.8–5.0	Metallic	Pale brass-yellow	Greenish black to brownish black	Indistinct; conchoidal to uneven fracture	Cubes with fine striations (lines on adjacent faces run at right angles), pyritohedra, octahedra, framboids, concretions, flat radiating discs ("pyrite suns"); brittle; sparks against steel; sulfur smell when struck or heated; too hard for a knife; weathers to limonite pseudomorphs	Nearly every environment: hydrothermal veins, black shale and coal (sedimentary), metamorphic rocks. Navajún, Spain (perfect cubes); Huanzala, Peru; Elba, Italy	"Fool's gold", yet the most common host of "invisible" gold: refractory ores hold "invisible" gold in pyrite, released by roasting or oxidation; source of sulfur and sulfuric acid; not used as iron ore; name from Greek pyr, "fire" (sparks against steel)
Sphalerite	ZnS, usually (Zn,Fe)S	Sulfide (sphalerite group)	Isometric	3.5–4	3.9–4.1	Resinous to adamantine; submetallic when iron-rich and black	Yellow, honey-brown, red ("ruby jack"), black (iron-rich "blackjack"), green	White to pale yellow or light brown (much lighter than the specimen)	Perfect dodecahedral, 6 directions	Tetrahedral crystals, cleavable masses, botryoidal crusts; dissolves in HCl with H₂S (rotten-egg) smell; some fluoresce orange or yellow (e.g. Franklin, NJ); triboluminescent; very high dispersion (about 0.156, roughly three and a half times diamond's 0.044), so clear gem sphalerite shows more fire than diamond but is too soft to wear; name means "treacherous" (mistaken for galena)	Mississippi Valley-type carbonate-hosted deposits, hydrothermal veins, sedimentary exhalative deposits. Tri-State district; Elmwood, Tennessee; Picos de Europa, Spain (gemmy)	Main ore of zinc (galvanizing steel, brass, die-casting, batteries); by-product cadmium, gallium, germanium, indium
Stibnite*	Sb₂S₃	Sulfide	Orthorhombic	2	4.6–4.7	Metallic, splendent on fresh faces	Lead-gray to steel-gray; tarnishes black or iridescent	Lead-gray to black	Perfect, 1 direction (lengthwise); subconchoidal fracture	Elongated prismatic to acicular or bladed crystals, striated lengthwise, often bent or twisted; radiating sprays; slightly sectile, slightly flexible; fuses easily (melts in a match or candle flame)	Low-temperature hydrothermal veins and hot-spring deposits. Ichinokawa mine, Japan; Xikuangshan, Hunan, China; Baia Sprie, Romania	Main ore of antimony (flame retardants, lead-acid battery and type-metal alloys); name from Latin stibium (source of the symbol Sb)

Halides

ReferenceHalides: reference table· 2 rows

Halite is also the answer for the rock "Rock Salt (Halite)" on the list.

2 rows

Mineral	Formula	Class & group	Crystal system	H	SG	Luster	Color	Streak	Cleavage / fracture	Habit & diagnostic tests	Forms in & notable localities	Uses, ores & gem varieties
Fluorite	CaF₂	Halide	Isometric	4 (Mohs standard)	3.18	Vitreous	Purple, green, blue, yellow, colorless, pink; often color-zoned or banded (e.g. Blue John)	White	Perfect octahedral, 4 directions (cleaves into octahedra)	Cubes, octahedra, interpenetrating twins; fluorescence is named after it, typically blue to violet under LW (and often SW) UV; some varieties thermoluminescent or phosphorescent; brittle; no fizz in acid	Hydrothermal veins, Mississippi Valley-type deposits, granite pegmatites and greisens. Illinois–Kentucky Fluorspar District (Cave-in-Rock); Weardale, England; Castleton, Derbyshire (Blue John)	Flux in steelmaking and aluminum smelting ("fluor" = to flow); source of hydrofluoric acid (fluorochemicals, refrigerants, Teflon); optical lenses; ornamental carving
Halite	NaCl	Halide	Isometric	2–2.5	2.17	Vitreous	Colorless, white; also pink, orange, blue (radiation damage)	White	Perfect cubic, 3 directions at 90°; conchoidal fracture	Cubes, hopper (skeletal) crystals, granular masses; salty taste; dissolves in water; can draw in moisture from humid air and turn damp (hygroscopic, mostly from Mg and Ca chloride impurities); some fluoresce red to orange under SW UV (manganese or organic inclusions)	Evaporite deposits from seawater or salt lakes; salt domes; playas. Wieliczka, Poland; Searles Lake, California; Stassfurt, Germany	Table salt, road de-icing salt, chlor-alkali industry (chlorine, sodium hydroxide), food preservation, water softening; name from Greek hals, "salt"; historically a major traded commodity

Oxides and hydroxides

ReferenceOxides and hydroxides: reference table· 7 rows

* = State/National only. "Trigonal" belongs to the hexagonal crystal family.

7 rows

Mineral	Formula	Class & group	Crystal system	H	SG	Luster	Color	Streak	Cleavage / fracture	Habit & diagnostic tests	Forms in & notable localities	Uses, ores & gem varieties
Corundum	Al₂O₃	Oxide (hematite group)	Trigonal	9 (Mohs standard)	3.9–4.1	Adamantine to vitreous	Gray, brown, blue (sapphire), red (ruby), also pink, yellow, colorless	White (harder than streak plate)	none; basal and rhombohedral parting; conchoidal to uneven fracture	Barrel-shaped hexagonal prisms and tapering dipyramids; striations on basal faces; heavy for a nonmetallic; star stones (asterism) from rutile needles; ruby fluoresces red under LW and SW UV (chromium)	Silica-poor metamorphic rocks (marble, aluminous schist), syenite and nepheline syenite pegmatites, placer gravels. Mogok, Myanmar (ruby); Sri Lanka (sapphire); Kashmir, India (sapphire)	Gem varieties ruby (red, Cr) and sapphire (all other colors, blue from Fe + Ti); abrasive (emery = corundum + magnetite/hematite); refractory (inert, melts above 2,000 °C: furnace linings, crucibles, alumina spark-plug insulators); synthetic corundum for watch crystals, jewel bearings, scratch-resistant phone and camera covers, and lasers; rutile "silk" marks a natural, unheated stone; name from Tamil kuruntam
Goethite/Limonite	Goethite α-FeO(OH); limonite FeO(OH)·nH₂O	Hydroxide (goethite is a defined species; limonite is a field name for poorly crystalline mixtures of hydrated iron oxides, mostly goethite)	Goethite orthorhombic; limonite amorphous (no crystal system)	Goethite 5–5.5; limonite 4–5.5 (earthy masses feel much softer)	Goethite 3.3–4.3; limonite 2.7–4.3	Goethite adamantine to submetallic, silky when fibrous; limonite earthy to dull	Dark brown to black (goethite); yellow-brown to rusty brown (limonite)	Yellowish brown to brown (both)	Goethite perfect, 1 direction; limonite none, earthy or uneven fracture	Goethite: botryoidal or stalactitic with radiating fibrous interior, velvety needle crusts; limonite: earthy, ochreous, concretions, rusty pseudomorphs after pyrite; neither is magnetic until heated in a reducing flame, which turns them to magnetite	Weathering of iron minerals: oxidized zones and gossans ("iron hats"), bog iron, laterites. Lake Superior iron ranges; Pikes Peak, Colorado (goethite); Lostwithiel, Cornwall	Minor ore of iron; yellow ochre pigment; gossans guide prospectors to buried sulfide ore; goethite named after poet J. W. von Goethe
Hematite	Fe₂O₃	Oxide (hematite group)	Trigonal	5–6	5.0–5.3	Metallic (specular) to dull/earthy	Steel-gray to black (specular, kidney ore); red to red-brown (earthy, oolitic)	Red to reddish brown ("cherry red")	none; uneven to subconchoidal fracture	Botryoidal/reniform ("kidney ore"), micaceous plates (specularite), rosettes ("iron roses"), oolitic, earthy, tabular crystals; red-brown streak is decisive whatever the color; not attracted by an ordinary magnet (at most very weakly by a strong one); becomes strongly magnetic after heating in a reducing flame (turns to magnetite)	Banded iron formations (Precambrian sedimentary; mostly 2.6–1.8 Ga, oldest about 3.8 Ga, a brief return about 0.75 Ga), hydrothermal veins, oxidation of magnetite, red beds. Mesabi Range, Minnesota; Hamersley Range, Western Australia; Cumbria, England (kidney ore)	Chief ore of iron (steel); red ochre pigment; jeweller's rouge (polishing); "hematine" jewellery beads are often synthetic; name from Greek haima, "blood" (red powder)
Magnetite	Fe₃O₄ (Fe²⁺Fe³⁺₂O₄)	Oxide (spinel group)	Isometric	5.5–6.5	5.2	Metallic to submetallic	Black	Black	none; octahedral parting; uneven to subconchoidal fracture	Octahedra, granular masses, black sand; strongly magnetic (attracts a magnet); lodestone is naturally magnetized and picks up iron filings	Accessory in igneous rocks (layered mafic intrusions), skarns, metamorphic rocks, banded iron formations, placer black sands. Kiruna, Sweden; Bushveld Complex, South Africa; Magnet Cove, Arkansas	Major ore of iron; lodestone used in early compasses; records Earth's magnetic field (paleomagnetism); heavy-media separation; name probably from Magnesia, an ancient Greek region
Pyrolusite*	MnO₂	Oxide (rutile group)	Tetragonal	6–6.5 (crystals); 2–6 in massive or sooty forms	4.4–5.1	Metallic to dull/earthy	Black to dark steel-gray	Black to bluish black	Perfect, 2 directions (prismatic) in crystals; uneven fracture	Fibrous or columnar radiating aggregates, earthy and sooty masses, botryoidal; dendrites on rock faces (black fern-like pseudofossils, usually mixed Mn oxides); soils fingers	Oxidation of manganese minerals; bog, lake and shallow-marine deposits; manganese nodules. Kalahari Manganese Field, South Africa; Ilfeld, Germany	Principal ore of manganese (steel alloys); dry-cell and alkaline batteries; decolorizing glass ("glassmaker's soap"); name from Greek "fire" + "wash"
Rutile*	TiO₂	Oxide (rutile group)	Tetragonal	6–6.5	4.2–4.3	Adamantine to submetallic	Reddish brown, red, golden yellow, black	Pale brown to light yellowish brown	Good, prismatic; uneven to subconchoidal fracture	Striated prismatic crystals, hair-like needles (in rutilated quartz; silk and stars in ruby/sapphire), elbow (geniculate) twins, lattice-like networks (sagenite); very high refractive index	Accessory in high-grade metamorphic rocks and igneous rocks; pegmatites; quartz veins; heavy-mineral beach sands. Graves Mountain, Georgia; Minas Gerais, Brazil (in quartz); beach sands of Western Australia	Ore of titanium (with ilmenite): TiO₂ white pigment for paint, paper, plastics; titanium metal; welding-rod coatings; name from Latin rutilus, "reddish"
Zincite*	ZnO	Oxide (wurtzite group)	Hexagonal	4	5.4–5.7	Subadamantine to resinous	Deep red to orange-red, rarely yellow	Orange-yellow	Perfect basal, 1 direction; conchoidal fracture	Usually massive or granular grains in white calcite with black franklinite and green willemite; natural crystals rare; bright synthetic crystals (smelter by-products) are common; dissolves in HCl without fizzing	Metamorphosed zinc deposit in marble. Franklin and Sterling Hill, New Jersey (the only major locality)	Ore of zinc at Franklin; synthetic ZnO used in sunscreen, ointments, rubber, ceramics

Carbonates and borates

ReferenceCarbonates and borates: reference table· 9 rows

* = State/National only; ¹ = supervisor supplies diagnostic information. Acid tests use cold dilute HCl unless stated.

9 rows

Mineral	Formula	Class & group	Crystal system	H	SG	Luster	Color	Streak	Cleavage / fracture	Habit & diagnostic tests	Forms in & notable localities	Uses, ores & gem varieties
Aragonite	CaCO₃	Carbonate (aragonite group; polymorph of calcite)	Orthorhombic	3.5–4	2.93–2.95	Vitreous; resinous on fractures	Colorless, white, gray, yellow, also blue, green, violet	White	Distinct, 1 direction (poor overall); subconchoidal fracture	Acicular needle sprays, columnar crystals, pseudohexagonal twinned prisms ("sixlings"), coral-like branches (flos ferri), pisolites; fizzes in cold dilute HCl like calcite; often fluoresces under UV; converts to calcite when heated (about 400 °C)	Low-temperature settings: shells, corals and pearls (nacre), cave formations, hot springs; high-pressure metamorphism (blueschist). Molina de Aragón, Spain (twins); Tazouta, Morocco; Erzberg, Austria (flos ferri)	Mother-of-pearl, pearls, coral reefs; cement and lime; aquarium substrate; named after Aragon, Spain
Azurite	Cu₃(CO₃)₂(OH)₂	Carbonate (with hydroxyl)	Monoclinic	3.5–4	3.7–3.8	Vitreous to adamantine; dull when earthy	Azure to deep blue	Light blue	Perfect, 1 direction, plus a fair one; conchoidal fracture; brittle	Prismatic or tabular crystals, rosettes, spherical nodules ("blueberries"), earthy crusts; fizzes in cold HCl; often intergrown with malachite (azurmalachite) and alters to malachite pseudomorphs	Oxidized zone of copper deposits. Bisbee, Arizona; Tsumeb, Namibia; Chessy, France ("chessylite")	Minor ore of copper; historic blue pigment; ornamental and collector specimens; name from Persian lazhward, "blue"
Calcite	CaCO₃	Carbonate (calcite group)	Trigonal	3 (Mohs standard)	2.71	Vitreous to pearly	Colorless, white; also yellow, orange, pink, green, blue, brown	White	Perfect rhombohedral, 3 directions not at 90° (about 75°/105°)	Scalenohedra ("dogtooth"), rhombs, flat "nailhead" crystals, stalactites, massive; fizzes vigorously in cold dilute HCl; strong double refraction in clear "Iceland spar"; many fluoresce (e.g. red under SW at Franklin, NJ)	Limestone and marble, cave deposits (travertine), hydrothermal veins, shells, carbonatites. Helgustaðir, Iceland (Iceland spar); Elmwood, Tennessee; Tri-State district (Joplin)	Cement and lime, building stone, agricultural lime, antacids, flux in steelmaking; polarizing prisms (Iceland spar); name from Latin calx, "lime"
Dolomite	CaMg(CO₃)₂	Carbonate (dolomite group)	Trigonal	3.5–4	2.84–2.86	Vitreous to pearly	White, pink, gray, colorless, tan	White	Perfect rhombohedral, 3 directions	Curved "saddle-shaped" rhombs, granular masses; fizzes only weakly in cold HCl unless powdered, readily in warm acid; some fluoresce	Dolomitization of limestone by Mg-rich fluids, evaporitic tidal flats, hydrothermal veins; dolomitic marble. Dolomites, Italy; Eugui, Spain; Tri-State district (Joplin)	Rock dolostone for building stone and aggregate; magnesium source; refractories; agricultural lime; flux; named after French geologist Déodat de Dolomieu
Malachite	Cu₂CO₃(OH)₂	Carbonate (with hydroxyl)	Monoclinic	3.5–4	3.6–4.0	Adamantine to vitreous; silky when fibrous; dull when earthy	Bright to dark green, in concentric light/dark bands	Light green	Perfect, 1 direction, rarely seen; uneven to splintery fracture	Botryoidal and stalactitic masses with concentric banding, fibrous aggregates, rare acicular crystals, pseudomorphs after azurite; fizzes in cold HCl	Oxidized zone of copper deposits. Katanga Copperbelt, DR Congo; Ural Mountains, Russia; Bisbee, Arizona	Ornamental and lapidary stone (banded cabochons, inlays); minor ore of copper; historic green pigment; name from Greek malache, "mallow" (leaf-green color); its copper-bearing dust is toxic, so wet-cut it and do not inhale or swallow it
Magnesite*¹	MgCO₃	Carbonate (calcite group)	Trigonal	3.5–4.5	2.98–3.0	Vitreous; dull and porcelain-like when massive	White, gray, yellowish, brown	White	Perfect rhombohedral (in crystals); massive form has conchoidal fracture	Usually massive, chalky or porcelain-like nodules with cauliflower-like surfaces; rare rhombs; fizzes only in warm HCl or when powdered	Alteration of serpentinite and other ultramafic rocks by CO₂-rich fluids; replacement of dolomite/limestone; evaporites. Brumado, Bahia, Brazil (crystals); Veitsch, Austria; Liaoning, China	Refractory bricks (as MgO); source of magnesium metal and compounds; often dyed to imitate turquoise
Rhodochrosite*	MnCO₃	Carbonate (calcite group)	Trigonal	3.5–4	3.5–3.7	Vitreous to pearly	Rose-pink to raspberry red; banded pink and white in stalactitic material	White	Perfect rhombohedral, 3 directions	Rhombohedral crystals, banded stalactites and botryoidal crusts, massive; fizzes in warm HCl (weakly in cold); surfaces blacken as Mn oxides form	Hydrothermal silver–lead–zinc–copper veins; sedimentary manganese deposits. Sweet Home Mine, Colorado (first worked for silver, when miners discarded rhodochrosite as worthless); Capillitas, Argentina (banded "Inca rose"); N'Chwaning, South Africa	Ornamental and gem material (Colorado state mineral); minor ore of manganese; name from Greek "rose-colored"
Siderite*	FeCO₃	Carbonate (calcite group)	Trigonal	3.5–4.5	3.9–4.0	Vitreous to pearly or silky	Tan, yellow-brown, brown, gray-green	White (may brown as it oxidizes)	Perfect rhombohedral, 3 directions	Curved saddle-shaped rhombs, botryoidal masses, concretions (clay ironstone); fizzes in warm HCl; weakly attracted by a strong magnet, strongly magnetic after heating (turns to magnetite); heavy for a carbonate; weathers to limonite	Sedimentary ironstone concretions (coal measures), hydrothermal veins, banded iron formations. Erzberg, Austria; Panasqueira, Portugal; Morro Velho, Brazil	Ore of iron (historically important); name from Greek sideros, "iron"
Ulexite	NaCaB₅O₆(OH)₆·5H₂O	Borate (listed separately on the official list)	Triclinic	2–2.5 (fibrous aggregates seem softer)	1.9–2.0	Vitreous; silky or satiny in fibrous masses	White, colorless	White	Perfect, 1 direction; uneven fracture	Parallel-fiber veins that act as optical fibers: a cut slab projects the image underneath onto its top ("TV rock"); "cotton-ball" nodules; acicular crystals; very light	Evaporite deposits in desert playas and salt lakes, with borax. Boron (Kramer), California; Death Valley, California; Argentine and Chilean salars; Turkey	Source of boron: fiberglass insulation, borosilicate glass, detergents, fertilizers; named after chemist G. L. Ulex

Sulfates

ReferenceSulfates: reference table· 6 rows

The three gypsum varieties share the parent row's formula, H, SG and streak; Rock Gypsum on the list is identified as gypsum.

6 rows

Mineral	Formula	Class & group	Crystal system	H	SG	Luster	Color	Streak	Cleavage / fracture	Habit & diagnostic tests	Forms in & notable localities	Uses, ores & gem varieties
Barite	BaSO₄	Sulfate (barite group)	Orthorhombic	3–3.5	4.3–4.5	Vitreous to pearly	Colorless, white, gray, blue, yellow, brown; red-brown when sand-filled	White	Perfect basal plus good prismatic (3 directions); uneven fracture	Tabular and bladed crystals, crested aggregates, "barite roses" (sandy rosettes; Oklahoma state rock), concretions, massive; remarkably heavy for a light-colored mineral; green flame test (barium); does not fizz	Hydrothermal veins, Mississippi Valley-type deposits, sedimentary concretions and beds, hot springs. Norman, Oklahoma (roses); Cumbria, England; Elk Creek, South Dakota	Drilling mud weighting agent (main use); barium source; "barium meal" for X-ray imaging; paint and plastic filler; radiation shielding; name from Greek barys, "heavy"
Celestite (celestine)	SrSO₄	Sulfate (barite group)	Orthorhombic	3–3.5	3.95–4.0	Vitreous to pearly	Pale sky-blue, colorless, white	White	Perfect basal plus good prismatic (3 directions); uneven fracture	Tabular to prismatic crystals, geodes lined with blue crystals, fibrous veins, nodules; crimson-red flame test (strontium); lighter than barite	Sedimentary: in limestone, dolostone and evaporites (with gypsum, anhydrite, sulfur). Sakoany, Madagascar (geodes); Sicily, Italy (with sulfur); Put-in-Bay, Ohio (Crystal Cave)	Main ore of strontium: red fireworks and flares, ferrite magnets, formerly TV-tube glass; name from Latin caelestis, "heavenly" (sky-blue)
Gypsum (parent)	CaSO₄·2H₂O	Sulfate (hydrated)	Monoclinic	2 (Mohs standard)	2.3	Vitreous to pearly on cleavage; silky when fibrous; dull when massive	Colorless, white, gray, pink, tan	White	Perfect, 1 direction (plus 2 lesser); thin sheets flexible but not elastic	Scratched by a fingernail; tabular crystals, "swallowtail" twins, sandy desert roses, curved "ram's horn" growths; does not fizz in acid; some specimens fluoresce	Evaporites (marine, sabkha, salt lakes), caves, oxidized zones where sulfide-rich water meets limestone. Naica, Mexico (Cave of the Crystals); White Sands, New Mexico (dune sand); Permian Basin, USA	Drywall (wallboard) and plaster of Paris (heated gypsum); cement set retarder; soil conditioner and fertilizer; blackboard chalk; name from Greek gypsos, "plaster"
↳ Alabaster (massive)	CaSO₄·2H₂O	Sulfate; gypsum variety	Monoclinic	2	2.3	Dull to sugary or subvitreous	White, cream, pink; translucent edges	White	Cleavage not visible; granular to uneven fracture	Fine-grained massive gypsum; easily scratched by a fingernail; feels warm; no fizz ("Egyptian alabaster" is really banded calcite, which fizzes)	Massive beds in evaporite sequences. Volterra, Tuscany, Italy; Fauld, Staffordshire, England	Carving, sculpture, translucent lamps and vases
↳ Satin spar (fibrous)	CaSO₄·2H₂O	Sulfate; gypsum variety	Monoclinic	2	2.3	Silky	White, pink, orange	White	Splits along fibers; splintery	Parallel fibers set across veins; chatoyant sheen when polished; scratched by a fingernail; no fizz; no "TV" image (unlike ulexite)	Fracture-filling veins in mudstone and shale of evaporite sequences, e.g. Mercia Mudstone, England	Beads and carvings; often sold under the wrong name "selenite" (sticks, towers, lamps)
↳ Selenite (crystalline)	CaSO₄·2H₂O	Sulfate; gypsum variety	Monoclinic	2	2.3	Vitreous to pearly	Colorless, transparent; may contain sand inclusions (hourglass patterns)	White	Perfect, 1 direction: peels into clear, flexible sheets	Transparent tabular or bladed crystals, swallowtail twins; giant crystals at Naica; scratched by a fingernail	Evaporites, caves, clay beds, salt plains. Naica, Mexico; Great Salt Plains, Oklahoma (hourglass selenite)	Display specimens; thin sheets used as window panes in Roman times; name from Greek selene, "moon"

Phosphates

ReferencePhosphates: reference table· 4 rows

* = State/National only. Vanadinite is a vanadate, grouped with the phosphates (apatite group) on the official list.

4 rows

Mineral	Formula	Class & group	Crystal system	H	SG	Luster	Color	Streak	Cleavage / fracture	Habit & diagnostic tests	Forms in & notable localities	Uses, ores & gem varieties
Apatite group / Fluorapatite	Group Ca₅(PO₄)₃(F,Cl,OH); fluorapatite Ca₅(PO₄)₃F	Phosphate (apatite group; fluorapatite is the commonest member)	Hexagonal	5 (Mohs standard)	3.1–3.2	Vitreous to subresinous	Green, blue-green, yellow, brown, purple, colorless	White	Poor basal; conchoidal to uneven fracture; brittle	Hexagonal prisms with flat or pyramidal ends, granular masses, sedimentary phosphorite; a steel knife just scratches it (H 5); some specimens fluoresce (e.g. yellow or orange) or are thermoluminescent; name from Greek "to deceive"; no fizz	Accessory mineral in almost all igneous rocks; pegmatites, carbonatites, metamorphic rocks; marine phosphorite beds; bones and teeth (hydroxylapatite). Cerro de Mercado, Durango, Mexico; Khibiny, Russia; phosphorite of Morocco and Florida	Phosphate rock (sedimentary apatite) is the essential, irreplaceable source of fertilizer phosphorus (the P in N-P-K) for phosphorus-deficient soils, and of phosphoric acid (main use); minor gemstone; tooth enamel and bone (hydroxylapatite; fluoride makes acid-resistant fluorapatite)
Pyromorphite*	Pb₅(PO₄)₃Cl	Phosphate (apatite group)	Hexagonal	3.5–4	6.5–7.1	Resinous to subadamantine	Green, yellow-green, yellow, brown, orange	White	Poor; uneven to subconchoidal fracture	Barrel-shaped hexagonal prisms, often hollow or cavernous at the ends; crusts and globular aggregates; very heavy for a nonmetallic; name means "fire form" (melted bead recrystallizes)	Oxidized zone of lead deposits (secondary after galena). Bunker Hill Mine, Idaho; Daoping, Guangxi, China; Les Farges, France	Minor ore of lead; collector specimens
Vanadinite*	Pb₅(VO₄)₃Cl	Vanadate (apatite group): lead chlorovanadate	Hexagonal	2.5–4	6.8–7.1	Resinous to subadamantine	Bright red, orange-red, brownish red, yellow	White to pale yellow	none; uneven to conchoidal fracture; brittle	Short, flat-topped hexagonal prisms (sometimes hollow), crusts; vivid red-orange color plus very high SG for a nonmetallic is decisive	Oxidized zone of lead deposits in arid regions. Mibladen, Morocco; Apache Mine and Old Yuma Mine, Arizona	Ore of vanadium (tough steel alloys; the Ti–6Al–4V titanium alloy of jet engines and airframes) and minor lead; collector specimens; named for its vanadium (after goddess Vanadís)
Turquoise*	CuAl₆(PO₄)₄(OH)₈·4H₂O	Phosphate (turquoise group)	Triclinic	5–6	2.6–2.9	Waxy to subvitreous; dull when porous	Sky-blue, blue-green, apple-green; often with brown/black "spiderweb" matrix veins	White to pale greenish blue	Perfect (rarely seen; crystals are tiny); conchoidal fracture	Cryptocrystalline massive nodules, veins and crusts; porous (absorbs oils and changes color; often stabilized); no fizz; dyed howlite or magnesite are common imitations	Secondary: meteoric water reacting with aluminous rocks near copper deposits in arid climates. Nishapur, Iran; Sinai, Egypt; Arizona and Nevada, USA	Gemstone and ornamental stone since antiquity; name from French for "Turkish"

Metallic minerals at a glance

ReferenceMetallic minerals at a glance· 15 rows

Every metallic or submetallic mineral on the list, sorted by streak (dark to light). No silicate on the list is metallic. * = State/National only.

15 rows

Mineral	Streak	H	SG	One decisive test
Magnetite	Black	5.5–6.5	5.2	Strongly attracted to a magnet
Pyrolusite*	Black to bluish black	2–6.5 (soft when sooty)	4.4–5.1	Soils fingers; fibrous or dendritic; not magnetic
Graphite	Black to dark gray	1–2	2.1–2.3	Greasy feel, marks paper, very light
Bornite*	Grayish black	3–3.25	5.1	Iridescent purple-blue tarnish over coppery fresh surface
Galena	Lead-gray	2.5–2.75	7.4–7.6	Perfect cubic cleavage, very heavy
Stibnite*	Lead-gray to black	2	4.6–4.7	Bladed, lengthwise-striated, often bent crystals; one cleavage
Chalcopyrite	Greenish black	3.5–4	4.1–4.3	Brassy and scratched by a knife
Pyrite	Greenish to brownish black	6–6.5	4.8–5.0	Brassy but not scratched by a knife; striated cubes
Hematite	Red to reddish brown	5–6	5.0–5.3	Red-brown streak from a gray or black specimen
Goethite/Limonite	Yellowish brown	4–5.5	2.7–4.3	Yellow-brown streak; rusty or fibrous botryoidal
Rutile*	Pale brown	6–6.5	4.2–4.3	Striated reddish prisms or needles; adamantine–submetallic
Sphalerite (dark, submetallic)	White to pale yellow-brown	3.5–4	3.9–4.1	Light streak from a dark specimen; 6 cleavages
Copper	Copper-red, shiny	2.5–3	8.9	Copper color, malleable, green or black tarnish
Gold	Golden yellow, shiny	2.5–3	19.3	Golden streak and malleable; enormous SG
Silver	Silver-white, shiny	2.5–3	10.5	Black tarnish; malleable wires

Silicates: amphiboles to kyanite

ReferenceSilicates (amphibole group to kyanite): reference table· 16 rows

The silicate tables follow list order; the "Class & group" column starts with the silicate structure (neso-, soro-, cyclo-, ino-, phyllo-, tectosilicate) because that is the most common written silicate question. * = State/National only. H = Mohs hardness; SG = specific gravity; LW/SW = long-/short-wave UV; XPL = crossed polars (thin section).

16 rows

Mineral	Formula	Class & group	Crystal system	H	SG	Luster	Color	Streak	Cleavage / fracture	Habit & diagnostic tests	Forms in & notable localities	Uses, ores & gem varieties
Actinolite*	Ca₂(Mg,Fe²⁺)₅Si₈O₂₂(OH)₂	Inosilicate, double chain; amphibole group (calcic amphibole; middle of tremolite–ferro-actinolite series)	Monoclinic	5–6	3.0–3.2	Vitreous; silky when fibrous	Bright to dark green (more Fe²⁺ = darker); gray-green	White	2 perfect at ~56° and ~124°; uneven to splintery	Long prismatic to acicular crystals, bladed, radiating sprays, fibrous/asbestiform; diamond-shaped cross-section; greener than tremolite because of Fe	Greenschist-facies metabasalt (greenstone) with chlorite, epidote, albite; contact-metamorphosed dolomitic limestone. Chester, Vermont; Zillertal, Austria	Nephrite jade (tough felted mat of tremolite–actinolite fibers); fibrous form is a regulated asbestos; collector specimens; name from Greek aktis, "ray" (radiating crystals)
Hornblende	(Ca,Na)₂₋₃(Mg,Fe,Al)₅(Si,Al)₈O₂₂(OH)₂ (e.g. Ca₂(Mg,Fe)₄Al(Si₇Al)O₂₂(OH)₂)	Inosilicate, double chain; amphibole group (calcic; "hornblende" is now a root name: magnesio-/ferro-hornblende)	Monoclinic	5–6	3.0–3.5 (rises with Fe)	Vitreous; silky sheen on cleavage	Black, dark green, dark brown	White to pale gray-green (lighter than specimen)	2 perfect at ~56° and ~124°; uneven	Elongate prismatic crystals with 6-sided (diamond-like) cross-section, columnar, bladed, granular; cleavage angle is the key test against augite (~87°/93°); Bowen discontinuous series after pyroxene, before biotite	Intermediate igneous rocks (diorite, andesite), some granites; main mineral of amphibolite and hornblende schist. Bancroft, Ontario; Arendal, Norway	Little direct use; rock-forming mineral of "black granite" dimension stone and amphibolite; indicates intermediate magma or amphibolite facies; name from German Horn + blenden, "deceive"
Tremolite*	Ca₂Mg₅Si₈O₂₂(OH)₂	Inosilicate, double chain; amphibole group (Mg end member of tremolite–actinolite series)	Monoclinic	5–6	2.9–3.1	Vitreous to silky	White, gray, colorless, pale green; lavender-pink (Mn-bearing "hexagonite")	White	2 perfect at ~56° and ~124°; uneven to splintery	Long prismatic, bladed, fibrous/asbestiform, radiating; some specimens fluoresce (variable); paler than actinolite (little or no Fe)	Metamorphism of siliceous dolomite: 5 dolomite + 8 quartz + H₂O → tremolite + 3 calcite + 7 CO₂; marbles and skarns. At higher temperature it reacts with calcite and quartz to diopside (a pyroxene), so tremolite marks a lower grade than diopside. Balmat–Edwards, St. Lawrence Co., NY; Campolungo, Switzerland	Nephrite jade (with actinolite); fibrous tremolite is a carcinogenic asbestos and a contaminant in some talc and vermiculite deposits; named after Val Tremola, Switzerland
Apophyllite*	KCa₄Si₈O₂₀(F,OH)·8H₂O (fluorapophyllite-(K), the common species)	Phyllosilicate (sheets of 4- and 8-membered rings); apophyllite group	Tetragonal	4.5–5	2.3–2.4	Vitreous; pearly on basal face ("fish-eye stone")	Colorless, white, pale green, pink	White	1 perfect basal; uneven fracture	Pseudo-cubic square prisms with pyramidal tips, tabular plates, druzy crusts; lines geodes and basalt vesicles with zeolites; flakes apart (exfoliates) when heated (name: Greek "away" + "leaf")	Low-temperature hydrothermal mineral in vesicles of basalt with stilbite, heulandite, calcite. Deccan Traps around Pune, Nashik and Jalgaon, India; Paterson, NJ; Nova Scotia	Collector specimens; rarely faceted (too soft and cleavable for wear)
Beryl	Be₃Al₂Si₆O₁₈	Cyclosilicate (6-membered rings); beryl group	Hexagonal	7.5–8	2.6–3.0 (usually ~2.7–2.8)	Vitreous	Colorless (goshenite), green, blue, yellow, pink, rarely red; color from trace ions (see gem table)	White	Imperfect basal; conchoidal to uneven	Long hexagonal prisms with flat basal (pinacoid) tops, lengthwise striations; giant crystals in pegmatite; flat top and H 7.5–8 separate it from quartz (pointed) and apatite (H 5)	Granitic pegmatites (late-stage Be-rich melt); emerald where Be fluids meet Cr/V-rich ultramafic rock or black shale. Muzo and Chivor, Colombia (emerald); Minas Gerais, Brazil (aquamarine); Black Hills, SD and Albany, Maine (giant crystals); Wah Wah Mtns, Utah (red beryl)	Ore of beryllium (light alloys, aerospace, X-ray windows); gems emerald, aquamarine, morganite, heliodor, goshenite, red beryl (bixbite)
Chrysocolla*	(Cu,Al)₂H₂Si₂O₅(OH)₄·nH₂O	Phyllosilicate (hydrated Cu silicate, poorly crystalline, often near-amorphous)	Orthorhombic (usually cryptocrystalline/amorphous)	2–4 (to ~7 when silicified with chalcedony)	1.9–2.4	Vitreous, waxy or earthy/dull	Sky-blue, cyan, blue-green (Cu²⁺)	White to pale blue-green	none; conchoidal to uneven	Crusts, botryoidal, massive, stalactitic, earthy; sticks to the tongue (porous); low SG and softness separate it from turquoise (H 5–6); no fizz in HCl, unlike malachite and azurite	Secondary (supergene) mineral in oxidized zones of copper deposits with malachite, azurite, cuprite, mostly in arid regions. Globe–Miami, Ray and Bisbee, Arizona; Chile; Katanga, DR Congo; Eilat, Israel	Minor copper ore; cabochons and carvings; "gem silica" = chalcedony colored by chrysocolla; name from Greek "gold glue" (old use as solder)
Dioptase*	Cu₆Si₆O₁₈·6H₂O (traditionally CuSiO₃·H₂O)	Cyclosilicate (6-membered rings)	Trigonal	5	3.3–3.4	Vitreous to subadamantine	Intense emerald-green to blue-green (Cu²⁺)	Pale green	Perfect rhombohedral, 3 directions; conchoidal to uneven; brittle	Short six-sided prisms with rhombohedral tips, druzy crusts in vugs; told from emerald by lower H (5), perfect cleavage, green streak and deeper color	Oxidized zones of copper deposits in arid climates. Tsumeb, Namibia; Altyn-Tyube, Kazakhstan; Mindouli, Republic of Congo; Christmas mine, Arizona	Collector specimens; occasionally cut, but too soft and cleavable for jewellery; name from Greek "to see through"
Epidote	Ca₂Al₂(Fe³⁺,Al)(SiO₄)(Si₂O₇)O(OH)	Sorosilicate (has both Si₂O₇ pairs and SiO₄); epidote group	Monoclinic	6–7	3.3–3.5	Vitreous	Pistachio-green, yellow-green, green-black (Fe³⁺)	Grayish white	1 perfect (parallel to the prism length), 1 imperfect; uneven	Prismatic crystals deeply striated along their length, columnar, radiating, granular crusts; strong pleochroism (green/brown/yellow); pistachio color is the field tell	Greenschist-facies metamorphism of mafic rocks (with chlorite, actinolite, albite); hydrothermal alteration of plagioclase; skarns. Knappenwand, Untersulzbachtal, Austria; Prince of Wales Island, Alaska; Pakistan	Indicator of low-grade metamorphism; unakite (granite altered to pink K-feldspar + green epidote + quartz) ornamental stone; collector specimens; name from Greek epidosis, "increase"
Albite	NaAlSi₃O₈	Tectosilicate; feldspar group, plagioclase series (Na end member, An₀–₁₀)	Triclinic	6–6.5	2.60–2.65	Vitreous; pearly on cleavage	White, colorless, gray, bluish sheen (peristerite)	White	2 good to perfect at ~94° (~86°); uneven to conchoidal	Tabular/platy crystals, "cleavelandite" = curved platy blades in pegmatites; polysynthetic albite twinning gives fine parallel striations on one cleavage face (plagioclase test); Carlsbad twins also; XPL: parallel black-and-white twin stripes	Granite, granitic pegmatite, spilite; greenschist-facies rocks; last (lowest-T) plagioclase on Bowen's continuous branch. Amelia Court House, Virginia (cleavelandite); Minas Gerais, Brazil; Swiss Alps (pericline)	Ceramic and glass flux (Na source); peristerite moonstone; Ab corner of the feldspar ternary diagram; name from Latin albus, "white"; plagioclase means "oblique break" (cleavages not quite 90°)
Labradorite	(Ca,Na)(Al,Si)₄O₈ (An₅₀–₇₀)	Tectosilicate; feldspar group, plagioclase series (calcic, An 50–70%)	Triclinic	6–6.5	2.68–2.72	Vitreous; pearly on cleavage	Gray, dark gray-blue to black; flashes of blue, green, gold, rarely full spectrum (labradorescence)	White	2 good to perfect at ~94° (~86°); uneven to conchoidal	Cleavable masses, rarely crystals; labradorescence = schiller from light interference in microscopic exsolution lamellae, visible only at certain angles; albite-twin striations on cleavage; higher-T plagioclase than albite (crystallizes earlier in Bowen)	Mafic igneous rocks: gabbro, basalt, anorthosite. Nain and Paul's Island, Labrador; Ylämaa, Finland (spectrolite); Madagascar; Oregon (sunstone, with Cu platelets)	Gem and ornamental stone (cabochons, tiles, countertops); spectrolite; Oregon sunstone (Oregon state gem); named after Labrador, Canada
Amazonite	KAlSi₃O₈	Tectosilicate; feldspar group, K-feldspar (green microcline)	Triclinic	6–6.5	2.56–2.58	Vitreous; pearly on cleavage	Bright green to blue-green; color linked to trace Pb and structural water, activated by natural irradiation; heating bleaches it	White	2 perfect at ~90°; uneven	Blocky prismatic crystals; white streaks/patches of albite (perthite) across the green; Carlsbad and Baveno twins; XPL: tartan (cross-hatched) twinning of microcline	Granitic pegmatites and amazonite granites, often with smoky quartz. Pikes Peak batholith, Colorado; Ilmen Mtns, Russia; Minas Gerais, Brazil; Madagascar	Ornamental and gem (beads, cabochons); named after the Amazon River (not found there)
Orthoclase/Microcline	KAlSi₃O₈	Tectosilicate; feldspar group, K-feldspar (alkali feldspar); polymorphs	Orthoclase monoclinic; microcline triclinic	6 (Mohs index); microcline 6–6.5	2.55–2.63	Vitreous; pearly on cleavage	Pink, salmon, flesh, tan, cream, white (pink/red from fine hematite)	White	2 perfect at ~90° (orthoclase = "straight break"; microcline slightly off 90°); uneven to conchoidal	Blocky prisms, phenocrysts; Carlsbad penetration twins (common), Baveno and Manebach twins; perthite (wavy white albite lamellae) instead of straight plagioclase striations; XPL: microcline tartan twinning, orthoclase untwinned or simple twin	Felsic rocks: granite, syenite, rhyolite, pegmatite, gneiss; arkose sandstone; late (low-T) on Bowen's series. Pikes Peak, Colorado; Karlovy Vary (Carlsbad), Czech Republic; Baveno, Italy	Ceramics, porcelain and glaze, glass (Al and alkali source); mild abrasive in scouring powder; moonstone (adularescent orthoclase–albite); rules: pink/tan feldspar = "Orthoclase/Microcline" or "Potassium feldspar"; orthoclase means "straight break", microcline "small slope"; feldspar from German "field spar"; North Carolina (Spruce Pine) leads US feldspar production
Almandine	Fe₃Al₂(SiO₄)₃	Nesosilicate; garnet group (pyralspite series, Fe²⁺–Al end member)	Isometric	7–7.5	4.1–4.3	Vitreous to resinous	Deep red, purplish red, brownish red to near-black (Fe²⁺)	White	none (parting possible); conchoidal to uneven	Dodecahedra (12 rhombic faces), trapezohedra (24 faces) or both; equant porphyroblasts in schist; weakly attracted to a strong (Nd) magnet; XPL: isotropic (stays black), high relief, no cleavage	Medium-grade regional metamorphism of shale (garnet zone): garnet schist, gneiss; some granites and pegmatites; placer sands. Wrangell (Stikine River), Alaska; Gore Mountain, NY; Emerald Creek, Idaho (star garnet); Roxbury, Connecticut	Abrasive (sandpaper, sandblasting, waterjet cutting); gem; index mineral; synthetic garnets such as YAG are not silicates; Mg²⁺ replaces Fe²⁺ toward pyrope; name from Alabanda, an ancient Anatolian city; "garnet" from Latin for pomegranate
Grossular (green)	Ca₃Al₂(SiO₄)₃	Nesosilicate; garnet group (ugrandite series, Ca–Al end member)	Isometric	6.5–7.5	3.4–3.7 (pure ~3.6)	Vitreous to resinous	Colorless when pure; green (V³⁺, Cr³⁺; tsavorite); also yellow, orange-brown (hessonite, Fe/Mn), pink	White	none; conchoidal to uneven	Dodecahedra and trapezohedra; massive (hydrogrossular "Transvaal jade"); lower SG than almandine; green color plus garnet shape separates it from olivine and epidote	Contact-metamorphosed impure limestone (skarn) and rodingite; tsavorite in graphitic gneiss of East Africa. Jeffrey mine, Asbestos, Quebec; Sierra de Cruces, Mexico; Tsavo, Kenya; Merelani, Tanzania	Gems tsavorite (green) and hessonite (orange "cinnamon stone"); collector specimens; name from Latin grossularia, "gooseberry" (pale green)
Kaolinite	Al₂Si₂O₅(OH)₄	Phyllosilicate, 1:1 (T-O) clay; kaolinite–serpentine group	Triclinic	2–2.5	2.6–2.7	Earthy/dull; pearly in crystal plates	White, cream, gray; tinted yellow, red by iron	White	Perfect basal (only microscopic); earthy, uneven fracture	Massive, earthy, chalky clay; microscopic pseudo-hexagonal plates and "books"; earthy smell when breathed on or wet; plastic when wet; sticks to tongue; no fizz in HCl (unlike chalk)	Chemical weathering (hydrolysis) of feldspar in warm humid climates (tropical and rainforest soils, laterites); hydrothermal alteration of granite. (Illite, a K-bearing mica-like clay off the list, is the commonest clay in marine and lake shales.) Jingdezhen, China; Cornwall, England (china clay); Georgia, USA	Porcelain, china and ceramics; coating and filler in glossy paper; paint, rubber, cosmetics; formerly in anti-diarrhea medicine (kaolin); name from Gaoling ("high ridge"), a Chinese village
Kyanite	Al₂SiO₅	Nesosilicate; Al₂SiO₅ polymorph group (with andalusite, sillimanite)	Triclinic	4.5–5 along the blade; 6.5–7 across	3.5–3.7	Vitreous; pearly on cleavage	Blue, often patchy (darker in center; Fe–Ti charge transfer); also white, gray, green, black	White	1 perfect, 1 good, plus parting; splintery	Long flat bladed crystals, single or in radiating sprays; directional hardness is diagnostic: a steel knife scratches it along the blade length (H 4.5–5) but not across it (H 6.5–7), because bonds differ in strength along the two directions; blue color plus blades separates it from sillimanite	Medium-to-high grade, high-pressure regional metamorphism of aluminous shale (kyanite zone): schist, gneiss, quartz veins. Pizzo Forno, Switzerland; Minas Gerais, Brazil; Willis Mountain, Virginia (major producer); Nepal	Refractories (firebrick, kiln furniture, spark-plug porcelain; turns to mullite and expands when fired); gem cabochons; index mineral for high pressure; name from Greek kyanos, "dark blue"

Silicates: micas to zircon

ReferenceSilicates (mica group to zircon): reference table· 17 rows

• = State/National only. H = Mohs hardness; SG = specific gravity; LW/SW = long-/short-wave UV; XPL = crossed polars (thin section).

17 rows

Mineral	Formula	Class & group	Crystal system	H	SG	Luster	Color	Streak	Cleavage / fracture	Habit & diagnostic tests	Forms in & notable localities	Uses, ores & gem varieties
Biotite	K(Mg,Fe)₃AlSi₃O₁₀(OH,F)₂	Phyllosilicate, 2:1 (T-O-T); mica group (trioctahedral; series annite–phlogopite)	Monoclinic	2.5–3	2.7–3.3	Vitreous to pearly; submetallic on fresh cleavage	Black, dark brown, dark green	White to gray	1 perfect basal; thin flexible, elastic sheets	Micaceous "books" of pseudo-hexagonal plates; flakes in rocks; peels into elastic sheets; dark color separates it from muscovite; weathered flakes look golden (mistaken for gold)	Granite, granodiorite, diorite, rhyolite, pegmatite; schist and gneiss (biotite zone); last mafic mineral on Bowen's discontinuous branch. Bancroft, Ontario; Mt Vesuvius, Italy	K–Ar and Ar–Ar dating; alters to vermiculite (horticulture, insulation); index mineral; poorer electrical insulator than muscovite (iron content), so little used as one; named after French physicist J.-B. Biot
Lepidolite*	K(Li,Al)₃(Si,Al)₄O₁₀(F,OH)₂	Phyllosilicate, 2:1; mica group (Li mica; series trilithionite–polylithionite)	Monoclinic	2.5–3 (to 4)	2.8–2.9	Pearly to vitreous	Lilac, pink, purple (Mn), also gray, colorless	White	1 perfect basal; flexible, elastic flakes	Fine scaly/granular masses, small micaceous books and rosettes; crimson lithium flame; lilac color plus mica cleavage is diagnostic (name from Greek "scale")	Lithium-rich granitic pegmatites with pink elbaite (rubellite), spodumene, amblygonite, albite, quartz. Pala, California; Black Hills, SD; Tanco mine, Manitoba; Minas Gerais, Brazil	Ore of lithium, rubidium and caesium; ornamental (carvings, tumbled stones)
Muscovite	KAl₂(AlSi₃O₁₀)(OH)₂	Phyllosilicate, 2:1; mica group (dioctahedral, K–Al white mica)	Monoclinic	2–2.5 on cleavage (to ~4 across)	2.8–2.9	Vitreous to pearly or silky	Colorless, silvery white, pale brown, pale green (Cr-rich fuchsite emerald-green)	White	1 perfect basal; thin transparent, flexible, elastic sheets	Large micaceous "books" with pseudo-hexagonal outline; fine silky sericite; splits into clear sheets; pale color vs biotite; resists weathering (detrital flakes in sandstone)	Granite, pegmatite (books up to meters), schist, phyllite, gneiss; late on Bowen's series (just before quartz); breaks down at sillimanite grade. Spruce Pine, North Carolina; Bihar and Andhra Pradesh, India; Minas Gerais, Brazil	Electrical insulators and capacitor dielectric, heater-element supports; stove and furnace windows ("isinglass"); joint compound, paint, roofing, cosmetic shimmer; fuchsite colors green aventurine; K–Ar dating; name from "Muscovy glass" (Russian window mica)
Olivine	(Mg,Fe)₂SiO₄	Nesosilicate; olivine group (forsterite Mg₂SiO₄–fayalite Fe₂SiO₄ solid solution)	Orthorhombic	6.5–7	3.2–4.4 (forsterite 3.2 → fayalite 4.4; common 3.3–3.4)	Vitreous	Olive green, yellow-green (Fe²⁺); brown when Fe-rich or weathered	White	Poor/indistinct; conchoidal fracture	Granular sugary masses, equant grains in basalt, mantle xenolith "nodules"; rarely crystals; first to crystallize on Bowen's series and first to weather; never with quartz in igneous rock; alters to serpentine and iddingsite	Mafic and ultramafic igneous rocks (basalt, gabbro, peridotite, dunite); dominant upper-mantle mineral; pallasite meteorites. San Carlos, Arizona; Zabargad (St John's) Island, Egypt; Papakōlea green-sand beach, Hawaii; Myanmar; Pakistan	Gem peridot; foundry and refractory sand; steel-slag conditioner; studied for CO₂ capture by weathering; named for its olive-green color
Prehnite*	Ca₂Al(AlSi₃O₁₀)(OH)₂	Phyllosilicate (structure transitional between sheet and chain silicates); prehnite group	Orthorhombic	6–6.5	2.8–2.95	Vitreous to pearly	Pale green, yellow-green (Fe³⁺), gray, white	White	Good basal; uneven	Botryoidal to globular crusts, rounded "bow-tie" or fan-shaped aggregates of tabular crystals; often with dark epidote needles inside; translucent apple-green with pearly surface	Low-temperature hydrothermal mineral in basalt cavities with zeolites; prehnite–pumpellyite facies metamorphism. Paterson, NJ; Lake Superior copper district; Kayes region, Mali; Cradock, South Africa	Cabochons and beads; collector specimens; named after Hendrik von Prehn
Augite	(Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)₂O₆	Inosilicate, single chain; pyroxene group (clinopyroxene)	Monoclinic	5.5–6	3.2–3.6	Vitreous to dull	Black, dark green, dark brown	Greenish gray to white	2 good at ~87° and ~93°; uneven to conchoidal	Short stubby prisms with nearly square, 8-sided cross-sections; granular; phenocrysts in basalt; XPL: high relief, two ~90° cleavages in end sections, inclined extinction (~35–48°), second-order colors, simple twins	Mafic igneous rocks (basalt, gabbro, diabase), some andesite and peridotite; high-grade granulite. Second on Bowen's discontinuous series. Mt Vesuvius and Etna, Italy; Eifel, Germany	Rock-forming only (part of basalt aggregate and "black granite"); thin-section mineral named in the rules; name from Greek auge, "lustre"
Rhodonite*	(Mn²⁺,Fe,Mg,Ca)SiO₃ (≈ MnSiO₃)	Inosilicate, single chain (pyroxenoid, 5-tetrahedron repeat); rhodonite group	Triclinic	5.5–6.5	3.5–3.8	Vitreous; pearly on cleavage	Rose-pink to red (Mn²⁺), commonly with black veins and dendrites of Mn oxide	White	2 perfect near 90° (~92.5°/87.5°); conchoidal to uneven; tough in masses	Massive, granular, rare tabular crystals; black veining is the classic look; scratches glass and does not fizz (vs rhodochrosite, H 3.5–4, fizzes, white banding)	Metamorphosed Mn-rich sediments, skarns, hydrothermal veins. Broken Hill, New South Wales; Franklin, NJ; Ural Mountains, Russia; Massachusetts (state gem)	Ornamental (carvings, beads, cabochons); minor manganese ore; name from Greek rhodon, "rose"
Spodumene*	LiAlSi₂O₆	Inosilicate, single chain; pyroxene group (clinopyroxene)	Monoclinic	6.5–7	3.0–3.2	Vitreous; pearly on cleavage	Grayish white, colorless, pink-lilac (kunzite, Mn), green (hiddenite, Cr), yellow	White	2 perfect at ~87°/93°, plus parting; splintery to uneven	Flattened lath-like prisms striated lengthwise, can be giant (12+ m logs, Etta mine); strong pleochroism (kunzite); kunzite fluoresces and phosphoresces pink-orange and fades in sunlight; crimson lithium flame	Lithium-rich granitic pegmatites with lepidolite, elbaite, amblygonite, albite, quartz. Etta mine, Keystone, SD; Pala, California (kunzite); Hiddenite and Kings Mountain, NC; Greenbushes, Western Australia; Nuristan, Afghanistan	Major ore of lithium (Li-ion batteries, glass-ceramics, greases); gems kunzite and hiddenite; name from Greek for "ash-colored"
Sodalite	Na₈(Al₆Si₆O₂₄)Cl₂	Tectosilicate; feldspathoid (sodalite group)	Isometric	5.5–6	2.27–2.33	Vitreous to greasy	Royal blue to violet-blue, often with white veins; also gray, white (blue from color centers)	White to very pale blue	Poor dodecahedral (6 directions); conchoidal to uneven	Massive granular, rare dodecahedra; often fluoresces orange under LW UV; hackmanite variety is tenebrescent (darkens to pink/violet under UV, then fades); low SG and white streak separate it from lapis lazuli (blue streak, pyrite flecks) and azurite (blue streak, fizzes, SG 3.8)	Silica-poor (undersaturated) igneous rocks, mainly nepheline syenite; never with primary quartz. Bancroft, Ontario ("Princess Blue"); Ice River, British Columbia; Litchfield, Maine; Bahia, Brazil; Namibia	Ornamental stone (carvings, beads, cabochons, tiles); named for its sodium
Staurolite	Fe²⁺₂Al₉Si₄O₂₃(OH)	Nesosilicate; staurolite group	Monoclinic (pseudo-orthorhombic)	7–7.5	3.7–3.8	Vitreous to resinous; dull on rough crystals	Reddish brown to dark brown, yellow-brown, black	White to grayish	Distinct, 1 direction (prismatic); subconchoidal to uneven	Stubby 6-sided prisms; cruciform penetration twins at ~60° ("St Andrew's cross", more common) and ~90° (Greek cross); rough-surfaced porphyroblasts in mica schist	Medium-grade regional metamorphism of aluminous shale (staurolite zone): mica schist with garnet and kyanite; survives in placers. Fannin Co., Georgia (state mineral); Patrick Co., Virginia (Fairy Stone State Park); Taos, New Mexico; Brittany, France	"Fairy cross" charms; minor sandblasting abrasive; index mineral for medium grade; name from Greek stauros, "cross"
Stilbite*	NaCa₄(Si₂₇Al₉)O₇₂·28H₂O (stilbite-Ca)	Tectosilicate; zeolite group	Monoclinic	3.5–4	2.1–2.2	Vitreous; pearly on cleavage	White, cream, peach, salmon-pink, orange	White	1 perfect (pearly cleavage face); uneven; brittle	Sheaf-like "bow-tie" or wheat-sheaf aggregates of thin tabular crystals, radiating balls; lines cavities with apophyllite and other zeolites; low SG	Low-temperature hydrothermal deposition in basalt vesicles and cavities. Deccan Traps (Pune, Nashik), India; Bay of Fundy, Nova Scotia (provincial mineral); Teigarhorn, Iceland; Paterson, NJ	Collector specimens; zeolites as a group are used as molecular sieves and ion exchangers (stilbite itself has little commercial use); name from Greek stilbein, "to shine" (pearly lustre)
Talc	Mg₃Si₄O₁₀(OH)₂	Phyllosilicate, 2:1 (T-O-T); talc–pyrophyllite group	Monoclinic or triclinic (polytypes)	1 (Mohs index)	2.6–2.8	Pearly to greasy/waxy; dull in masses	White, gray, pale green, brownish	White	1 perfect basal; flexible but inelastic flakes; uneven in masses	Foliated, micaceous, massive (steatite/soapstone), fibrous; soapy/greasy feel; scratched by fingernail; flakes bend and stay bent (mica springs back)	Metamorphism and hydrothermal alteration of ultramafic rocks (serpentinite → talc + carbonate) and of siliceous dolomite; talc schist, soapstone. Johnson, Vermont; Schuyler, Virginia (soapstone); Gouverneur, NY; Trimouns (Luzenac), France	Talcum and cosmetic powder; paper, paint and plastic filler; ceramics and steatite insulators; soapstone carvings, countertops, tailor's chalk; name from Arabic/Persian talq
Topaz	Al₂SiO₄(F,OH)₂	Nesosilicate; topaz group	Orthorhombic	8 (Mohs index)	3.4–3.6	Vitreous	Colorless, pale blue, yellow, golden-brown, sherry, pink, orange (imperial, Cr³⁺)	White	1 perfect basal (across the prism); subconchoidal to uneven	Prismatic crystals with lozenge (diamond) cross-section, lengthwise striations, wedge or pyramidal tips; basal cleavage cuts across the prism; scratches quartz; heavier than quartz and citrine of same size	Granitic pegmatites, gas cavities in rhyolite, greisen and high-T hydrothermal veins; placers. Ouro Preto, Minas Gerais (imperial); Thomas Range, Utah (state gem); Pikes Peak, Colorado; Ural Mountains; Katlang, Pakistan (pink)	Gem; blue topaz is irradiated and heated colorless topaz (Texas state gem: blue topaz); Mohs standard for 8
Tourmaline	XY₃Z₆(T₆O₁₈)(BO₃)₃V₃W; schorl NaFe²⁺₃Al₆(Si₆O₁₈)(BO₃)₃(OH)₄; elbaite Na(Li₁.₅Al₁.₅)Al₆(Si₆O₁₈)(BO₃)₃(OH)₄; dravite NaMg₃Al₆(Si₆O₁₈)(BO₃)₃(OH)₄	Cyclosilicate (6-membered rings; borosilicate); tourmaline group (list accepts "tourmaline")	Trigonal	7–7.5	2.9–3.3	Vitreous to resinous	Black (schorl, Fe), brown (dravite, Mg), every color in elbaite; color zoning (watermelon: pink core, green rind)	White (schorl may be grayish)	Indistinct; uneven to conchoidal	Long prisms with rounded triangular cross-section and deep lengthwise striations; different terminations at each end; radiating sprays and "suns", acicular; piezoelectric and pyroelectric (heated crystal attracts dust); strong pleochroism (darkest along c-axis); schorl weakly magnetic	Granitic pegmatites (elbaite), granites, schists and gneisses (dravite, schorl), hydrothermal veins. Minas Gerais, Brazil (widest color range); Paraíba, Brazil (Cu blue); Pala, California; Mount Mica, Maine (state mineral); Madagascar; Afghanistan	Gem: rubellite, indicolite, verdelite, watermelon, Paraíba; piezoelectric pressure gauges (historic); collector specimens; name from Sinhalese turamali, "mixed gems"
Willemite*	Zn₂SiO₄	Nesosilicate; phenakite group	Trigonal	5.5	3.9–4.2	Vitreous to resinous	White, colorless, green, yellow-green, brown, red-brown	White	Poor basal and prismatic; uneven to conchoidal	Massive granular, short hexagonal prisms (troostite), fibrous radiating, botryoidal; brilliant green fluorescence under SW UV, often also LW (Mn²⁺ activator), often phosphorescent; at Franklin with red-fluorescing calcite and black non-fluorescent franklinite	Metamorphosed zinc ore bodies; oxidized zones of zinc deposits. Franklin and Sterling Hill, NJ; Tsumeb, Namibia; La Calamine (Moresnet), Belgium (type locality)	Zinc ore (historically at Franklin); synthetic Mn-doped Zn₂SiO₄ was a green phosphor in early CRT and fluorescent lamps; named after King Willem I of the Netherlands
Wollastonite*	CaSiO₃	Inosilicate, single chain (pyroxenoid, 3-tetrahedron repeat); wollastonite group	Triclinic	4.5–5	2.8–3.1	Vitreous; silky or pearly on cleavage	White, gray, cream, colorless, pale green	White	1 perfect plus 2 good (at about 84°), all parallel to the fiber length; splintery to uneven	Fibrous, bladed, radiating masses, acicular; some specimens fluoresce (yellow-orange, variable); splintery white blades, softer than tremolite (H 5–6)	Contact metamorphism of siliceous limestone (skarn): CaCO₃ + SiO₂ → CaSiO₃ + CO₂ (at high T); carbonatites. Willsboro and Lewis, Adirondacks, NY; Crestmore, California; Franklin, NJ; China, India (major producers)	Ceramics (tiles, glazes; low shrinkage, fast firing); asbestos substitute (brake linings, fire boards); filler in plastics and paint; metallurgical flux; named after W. H. Wollaston
Zircon*	ZrSiO₄	Nesosilicate; zircon group	Tetragonal	7.5 (6–7.5 if metamict)	4.6–4.7 (metamict down to ~3.9)	Adamantine to vitreous; greasy when metamict	Brown, red-brown, yellow, colorless, green; blue and golden gems are heat-treated	White	Indistinct; conchoidal to uneven; brittle	Short square prisms with pyramids (dipyramidal), often doubly terminated; tiny grains in granite and detrital sand; high dispersion ("fire") and strong double refraction (doubled back facets); weakly radioactive (U, Th replace Zr) → radiation damage makes it metamict (lower SG, H; the lattice gradually becomes amorphous); zircon grains enclosed in biotite are ringed by dark pleochroic haloes of radiation damage; many fluoresce yellow under SW UV; heavy for a gem	Accessory mineral in granite, syenite, nepheline-syenite pegmatite, gneiss; resists weathering → heavy-mineral beach and placer sands. Jack Hills, Western Australia (oldest grains, 4.4 Ga); Ratanakiri, Cambodia; Sri Lanka; Myanmar; Bancroft, Ontario	Ore of zirconium (nuclear fuel cladding, zirconia ceramics, refractories, foundry sand) and hafnium; U–Pb dating (takes U, excludes Pb); gem; not to be confused with synthetic cubic zirconia (ZrO₂); name from Persian zargun, "gold-colored"

Quartz varieties

ReferenceQuartz varieties: reference table· 11 rows

• = State/National only. All quartz varieties are SiO₂, H ≈ 7, white streak and no cleavage; the Color column gives the cause of each variety's color. Opal is not crystalline quartz but is filed with the quartz varieties on the list.

Macrocrystalline vs microcrystalline. Macrocrystalline quartz grows as crystals or grains big enough to see: rock crystal, amethyst, citrine, smoky quartz, milky quartz, rose quartz (massive, but made of coarse quartz) and aventurine (a rock of visible quartz grains). Microcrystalline (cryptocrystalline) quartz is built of crystals or fibers too small to see without a microscope, so it is translucent to opaque and waxy to dull, never shows prism faces, and breaks with a smooth conchoidal surface: agate/chalcedony (with onyx and carnelian), jasper, and the rocks chert and flint. Tiger's eye, fibrous quartz intergrown with (traditionally said to replace) crocidolite asbestos fibers, is placed in either group depending on the source. Opal is neither: it is amorphous.

11 rows

Mineral	Formula	Class & group	Crystal system	H	SG	Luster	Color	Streak	Cleavage / fracture	Habit & diagnostic tests	Forms in & notable localities	Uses, ores & gem varieties
Aventurine	SiO₂ (+ platy inclusions)	Tectosilicate; quartz (silica) group; strictly a quartzite or quartz rock with inclusions	Trigonal (quartz grains)	6.5–7	2.64–2.69	Vitreous to waxy; glittering	Green from Cr-mica fuchsite platelets; red-brown/orange from hematite or goethite platelets; blue from dumortierite; the reflecting platelets give aventurescence (glitter)	White	none; conchoidal to uneven (granular)	Massive, granular; glittering spangles that flash as the stone is turned (aventurescence); no cleavage and H 7 separate it from amazonite and jade	Metamorphic quartzites and quartz rock with mica or hematite. Mysore and Bellary, India (most green aventurine); Brazil; Russia	Beads, carvings, tumbled stones, cabochons; man-made "goldstone" glass imitates it
Agate/Chalcedony	SiO₂	Tectosilicate; quartz (silica) group; microcrystalline (cryptocrystalline) quartz, often with moganite	Trigonal (microscopic fibers)	6.5–7	2.58–2.64	Waxy to vitreous; dull on rough surfaces	Color from microcrystalline fibrous structure scattering light (bluish-white); trace Fe oxides give red-orange carnelian, Ni gives green chrysoprase; agate = banded from rhythmic layers of silica gel; onyx = straight black-and-white bands	White	none; conchoidal to splintery	Botryoidal, mammillary, stalactitic, nodules; concentric or "fortification" banding in geodes, thundereggs and amygdules (geodic); moss agate has dendritic inclusions; translucent, waxy	Low-temperature silica from groundwater filling cavities in volcanic rocks (basalt vesicles, rhyolite) and in sediments. Hard, tough and inert, agates outlast their host rock, so weathered-out nodules travel far in rivers and glacial drift (Lake Superior agates turn up across the Midwest). Lake Superior (Minnesota state gem); Rio Grande do Sul, Brazil; Artigas, Uruguay; Botswana; Oregon thundereggs	Ornaments, beads, cameos (onyx, sardonyx), mortars and pestles, balance bearings; often dyed; Idar-Oberstein, Germany cutting center; agate named after the Achates River, Sicily; chalcedony after Chalcedon, an ancient town; bloodstone (heliotrope) is dark green chalcedony with red spots of jasper or iron oxide
Amethyst	SiO₂	Tectosilicate; quartz (silica) group	Trigonal	7	2.65	Vitreous	Purple to violet: Fe³⁺ replacing Si plus natural gamma irradiation creates Fe⁴⁺ color centers; heating to ~400–500 °C destroys them and turns it yellow-orange (most commercial citrine) or rarely green (prasiolite); can fade in strong sunlight	White	none; conchoidal (rippled fracture from twinning)	Short hexagonal prisms or just rhombohedral points lining geodes; druzy; color zoning concentrated in the tips (phantoms); Brazil-law twinning common; piezoelectric	Geodes in basalt flows; hydrothermal veins. Rio Grande do Sul, Brazil; Artigas, Uruguay; Thunder Bay, Ontario (provincial mineral); Four Peaks, Arizona; Zambia	Gem, traditionally the most valuable quartz; decorative geodes; heated to make citrine; name from Greek "not drunk"
Citrine	SiO₂	Tectosilicate; quartz (silica) group	Trigonal	7	2.65	Vitreous	Pale yellow to orange-brown; cause debated: trace iron (Fe³⁺/iron-oxide) or Al-related color centers in natural stones; heated amethyst is colored by iron oxides; rare natural citrine is pale, even yellow; most commercial citrine is heat-treated amethyst, often redder-orange with darker tips	White	none; conchoidal	Prismatic crystals with rhombohedral tips (natural); heated amethyst still has the geode-point shape; piezoelectric	Natural: granitic pegmatites and hydrothermal veins (Ural Mountains; Madagascar; DR Congo). Heated: from Brazilian and Uruguayan amethyst	Gem; cheaper substitute for yellow topaz (misnamed "Madeira topaz"); name from French citron, "lemon"
Jasper*	SiO₂ (+ up to ~20% impurities)	Tectosilicate; quartz (silica) group; opaque microgranular quartz/chalcedony	Trigonal (microscopic grains)	6.5–7	2.6–2.9	Dull; vitreous when polished	Red from hematite; yellow/brown from goethite (limonite); green from chlorite or other silicates; patterned or banded by mixed impurities; always opaque	White	none; conchoidal to uneven	Massive, nodular, banded or patterned (picture, orbicular jasper); opaque (chalcedony is translucent); sharp conchoidal edges	Silica precipitated in sediments, volcanic ash and hydrothermal veins; red bands in banded iron formation (jaspilite). Western Australia; Oregon and Idaho; India; Madagascar	Ornamental stone, cabochons, beads; ancient tools, seals and carvings (conchoidal fracture made sharp edges); name from Greek iaspis, "spotted stone"
Milky Quartz	SiO₂	Tectosilicate; quartz (silica) group	Trigonal	7	2.65	Vitreous to greasy	White, translucent to opaque: countless microscopic fluid (liquid and gas) inclusions scatter light	White	none; conchoidal	Massive vein quartz, crude prisms; white, greasy-looking masses; scratches glass	Hydrothermal quartz veins (often with gold, as in the Mother Lode, California); pegmatite cores. Worldwide	Aggregate, silica source for glass and silicon/ferrosilicon; guide to gold-bearing veins; ornamental
Opal	SiO₂·nH₂O (about 3–21% water, usually 6–10%)	Mineraloid (amorphous hydrated silica; opal-A or opal-CT); filed with quartz on the list	Amorphous (no crystal system)	5.5–6.5	1.9–2.3 (precious ~2.15)	Vitreous to resinous, waxy or pearly	Common opal ("potch") white, milky, gray, yellow, red-orange (fire opal, Fe oxides); precious opal shows play of color from diffraction of light by ordered layers of uniform silica spheres (~150–300 nm; sphere size sets the colors)	White	none; conchoidal	Massive, nodules, veinlets, botryoidal (hyalite), replacing wood, bone and shells (opalized fossils); low SG; many fluoresce (hyalite bright green under SW from uranyl); can craze (crack) when it dries	Low-temperature silica gel deposited from groundwater in weathered sediments and volcanic rocks. Coober Pedy, Andamooka, Lightning Ridge (black opal), Australia (most production); Querétaro, Mexico (fire opal); Wollo (Welo), Ethiopia; Virgin Valley, Nevada	Gem; diatomite (opaline diatom skeletons) for filters and mild abrasives, and historically to absorb and stabilize nitroglycerin in Nobel's dynamite; name from Sanskrit upala, "precious stone", via Latin opalus
Rock Crystal	SiO₂	Tectosilicate; quartz (silica) group	Trigonal (α-quartz; changes to hexagonal β-quartz above 573 °C)	7 (Mohs index)	2.65	Vitreous	Colorless, transparent: pure SiO₂ with no color centers or inclusions	White	none; conchoidal	Six-sided prisms with horizontal striations, capped by rhombohedra (pyramid-like); doubly terminated crystals in cavities (Herkimer "diamonds", NY); twins: Dauphiné, Brazil, Japan (V-shaped, ~84°); piezoelectric; weak double refraction (too small to see by eye, unlike calcite)	Hydrothermal veins and cavities, Alpine clefts, pegmatites. Hot Springs and Mount Ida, Arkansas; Herkimer County, NY; Swiss Alps; Minas Gerais, Brazil	Oscillators in watches and electronics (now mostly synthetic, hydrothermally grown); optics (UV-transparent lenses, prisms); crystal balls, jewellery; Mohs standard for 7; "crystal" from Greek krystallos, "ice"
Rose Quartz	SiO₂	Tectosilicate; quartz (silica) group	Trigonal	7	2.65	Vitreous to greasy	Pink, cloudy: microscopic fibrous inclusions of a dumortierite-like borosilicate scatter light; rare transparent pink crystals instead owe color to Al–P color centers and fade in light	White	none; conchoidal	Almost always massive (no crystal faces), translucent and hazy; oriented fibers can give a 6-rayed star (asterism) in spheres and cabochons	Cores of granitic pegmatites. Black Hills, South Dakota (state mineral); Minas Gerais, Brazil (crystals); Madagascar	Beads, carvings, spheres, ornamental stone
Smoky Quartz*	SiO₂	Tectosilicate; quartz (silica) group	Trigonal	7	2.65	Vitreous	Light brown, gray to black (morion): Al³⁺ replacing Si⁴⁺ plus natural irradiation creates hole color centers; heating bleaches it, irradiation makes it artificially	White	none; conchoidal	Well-formed prisms, often large, in pegmatites and Alpine clefts; often with amazonite (Pikes Peak)	Granites and pegmatites with slightly elevated radioactivity (K, U, Th); Alpine fissures. Pikes Peak and Crystal Peak, Colorado; Cairngorm Mtns, Scotland ("cairngorm", national gem); Swiss Alps; Brazil; New Hampshire (state gem)	Gem and ornamental stone
Tiger's Eye*	SiO₂ (+ iron oxides)	Tectosilicate; quartz (silica) group	Trigonal (fibrous intergrowth)	6.5–7	2.64–2.71	Silky	Golden yellow to brown, banded: quartz replacing or intergrown with parallel fibers of crocidolite (blue riebeckite asbestos); oxidation leaves goethite/limonite (gold-brown); the fibers give chatoyancy; unoxidized blue = hawk's eye; heated or hematitic = red tiger's eye	White	none; conchoidal to fibrous/splintery	Fibrous seams and veins with a moving silky light band (chatoyancy, cat's-eye) when cut as a cabochon across the fibers	Crack-seal veins in banded iron formation. Northern Cape (Griquatown, Prieska), South Africa; Hamersley Range, Western Australia (tiger iron); Namibia; Myanmar	Cabochons, beads, carvings; "tiger iron" (with hematite and red jasper)

Rules-named minerals not on the ID list

ReferenceMinerals named in the rules but not on the ID list· 4 rows

These are tested as index minerals, polymorphs or rock components, not as identification specimens.

4 rows

Mineral	Formula	Class & group	Crystal system	H	SG	Luster	Color	Streak	Cleavage / fracture	Habit & diagnostic tests	Forms in & notable localities	Uses, ores & gem varieties
Chlorite	(Mg,Fe)₃(Si,Al)₄O₁₀(OH)₂·(Mg,Fe)₃(OH)₆ (e.g. clinochlore (Mg₅Al)(AlSi₃)O₁₀(OH)₈)	Phyllosilicate, 2:1:1 (T-O-T layer + brucite-like interlayer); chlorite group	Monoclinic (most; some triclinic)	2–2.5	2.6–3.3	Vitreous, pearly or dull	Green (light to dark); rarely purple (Cr, kämmererite)	Pale green to greenish white	1 perfect basal; flexible but inelastic flakes	Scaly, foliated masses, micaceous, rosettes; green flakes that bend but do not spring back (micas do); gives slate, phyllite and greenschist their green color	Low-grade regional metamorphism (chlorite zone, greenschist facies) of shale and basalt; hydrothermal alteration of biotite, hornblende and pyroxene. Green Mountains, Vermont; Zermatt, Switzerland; Ural Mountains	Index mineral for low grade (first Barrovian zone); seraphinite (chatoyant clinochlore) ornamental; little industrial use; name from Greek chloros, "green"
Andalusite	Al₂SiO₅	Nesosilicate; Al₂SiO₅ polymorph group (with kyanite, sillimanite)	Orthorhombic	6.5–7.5	3.1–3.2	Vitreous	Pink, reddish brown, gray, green; gem andalusite strongly pleochroic (green/red-brown)	White	Good prismatic, 2 directions at ~89°; uneven to subconchoidal	Stubby prisms with nearly square cross-section; chiastolite variety shows a dark cross (X) of carbon/graphite inclusions in cross-section	Low-pressure metamorphism: contact aureoles in shale (hornfels, spotted slate) and Buchan-type regional metamorphism. Andalusia, Spain (name); Lancaster, Massachusetts and Bimbowrie, Australia (chiastolite); Minas Gerais, Brazil (gem)	Refractories (bricks, kiln furniture, spark plugs; converts to mullite); gem; low-pressure indicator; named after Andalusia, Spain
Sillimanite	Al₂SiO₅	Nesosilicate; Al₂SiO₅ polymorph group (with kyanite, andalusite)	Orthorhombic	6.5–7.5	3.2–3.3	Vitreous to silky	White, colorless, gray, pale brown, yellowish, greenish	White	1 perfect, lengthwise; uneven to splintery	Long slender prisms, needles and fibrous mats (fibrolite), parallel or radiating; silky fibers separate it from kyanite's broad blue blades	High-temperature metamorphism: sillimanite zone (upper amphibolite to granulite facies) gneiss and migmatite; inner contact aureoles. Chester, Connecticut; Clearwater area, Idaho; Sri Lanka and Myanmar (gem); named after Benjamin Silliman	Refractories and high-alumina ceramics (mullite), spark-plug porcelain; rare gem (cat's-eye sillimanite); index mineral for the highest Barrovian grade
Serpentine	Mg₃Si₂O₅(OH)₄ (antigorite, chrysotile, lizardite)	Phyllosilicate, 1:1 (T-O); kaolinite–serpentine group, serpentine subgroup	Monoclinic (antigorite, chrysotile); lizardite trigonal or monoclinic	2.5–4 (chrysotile ~2.5; antigorite 3.5–4)	2.5–2.7	Greasy, waxy, resinous; silky (chrysotile)	Light to dark green, yellow-green, black-green, often mottled like snakeskin; white fibers (chrysotile)	White	Antigorite: perfect basal (rarely visible); chrysotile none; conchoidal to splintery	Massive, platy (antigorite) or fibrous, silky asbestiform veins (chrysotile, fibers crossing the vein); smooth, slippery feel; makes the rock serpentinite	Low-temperature hydration (serpentinization) of olivine and pyroxene in peridotite and dunite at mid-ocean ridges, ophiolites and subduction zones. Thetford Mines and Asbestos, Quebec (chrysotile); New Idria, California; Belvidere Mountain, Vermont	Chrysotile asbestos (formerly insulation, brake linings, fireproofing; now restricted as a carcinogen); ornamental stone (verde antique, bowenite "new jade"); serpentinite is California's state rock; name from Latin serpens (green, mottled like snakeskin)

Gem varieties

ReferenceGem varieties: reference table· 23 rows

"Precious" and "semi-precious" follow the traditional trade convention, not a scientific classification: diamond, ruby, sapphire and emerald are the four precious stones (pearl, and historically amethyst and opal, are sometimes added); all others are semi-precious. Organic gems are not minerals. Birthstones are rarely tested; the column follows the modern US list, and "—" means the variety is not on it.

23 rows

Gem	Mineral species	Formula	Color and its cause	H	Birthstone (modern US list)	Notes
Emerald (precious)	Beryl	Be₃Al₂Si₆O₁₈	Green: trace Cr³⁺ and/or V³⁺ replacing Al	7.5–8	May	Nearly always has inclusions ("jardin") and is commonly oil-filled; Muzo and Chivor, Colombia; Zambia; Brazil
Aquamarine (semi-precious)	Beryl	Be₃Al₂Si₆O₁₈	Blue to blue-green: Fe²⁺ (greenish stones are heated to reduce Fe³⁺ and look bluer)	7.5–8	March	Large clean crystals from pegmatites; Minas Gerais, Brazil; Pakistan; Madagascar
Morganite (semi-precious)	Beryl	Be₃Al₂Si₆O₁₈	Pink to peach: Mn²⁺ (often also caesium-bearing)	7.5–8	—	Named after J. P. Morgan; granitic pegmatites; Madagascar; Minas Gerais; Pala, California
Heliodor/golden beryl (semi-precious)	Beryl	Be₃Al₂Si₆O₁₈	Yellow to golden: Fe³⁺	7.5–8	—	Colorless beryl is goshenite; red beryl (bixbite, Mn³⁺) from Utah; Ukraine; Namibia; Brazil
Peridot (semi-precious)	Olivine (forsterite-rich)	(Mg,Fe)₂SiO₄	Yellow-green to olive: Fe²⁺ is part of the formula (idiochromatic), not a trace impurity	6.5–7	August	Strong double refraction (doubled facets); San Carlos, Arizona; Zabargad Island, Egypt; Myanmar; Pakistan; also in pallasite meteorites
Ruby (precious)	Corundum	Al₂O₃	Red: trace Cr³⁺ replacing Al (Cr also makes it fluoresce red)	9	July	Star rubies from oriented rutile needles (asterism); Mogok, Myanmar; Mozambique; Thailand
Sapphire (precious)	Corundum	Al₂O₃	Blue: Fe²⁺–Ti⁴⁺ charge transfer; yellow: Fe³⁺ or color centers; pink: less Cr than ruby; padparadscha (pink-orange): Cr plus color centers/Fe; green: Fe; purple: Cr plus Fe–Ti; color-change: V; any non-red corundum gem is a sapphire ("fancy sapphire" for non-blue)	9	September	Star sapphire (rutile needles); Kashmir; Sri Lanka; Myanmar; Yogo Gulch, Montana; Madagascar
Amethyst (semi-precious; once counted precious)	Quartz	SiO₂	Purple: Fe³⁺ plus natural irradiation (color centers); heating turns it yellow	7	February	Was rare and precious until large Brazilian deposits were found in the 1800s; geodes in basalt
Citrine (semi-precious)	Quartz	SiO₂	Yellow to orange: iron (Fe³⁺, iron oxides) or Al-related color centers (debated); most is heat-treated amethyst	7	November	Substitute for yellow topaz
Rose quartz (semi-precious)	Quartz	SiO₂	Pink: microscopic dumortierite-like fibrous inclusions	7	—	Massive, cloudy; star rose quartz from oriented fibers
Tiger's eye (semi-precious)	Quartz	SiO₂ (+ goethite)	Golden-brown: iron oxides from altered crocidolite fibers; fibrous structure gives chatoyancy	6.5–7	—	Blue unaltered form = hawk's eye; Griquatown, South Africa
Opal (semi-precious; sometimes counted precious)	Opal (mineraloid)	SiO₂·nH₂O	Play of color: diffraction from ordered silica spheres; fire opal orange-red from Fe oxides	5.5–6.5	October	Amorphous, so a mineraloid; can crack if it dries; Australia (most production); Mexico; Ethiopia
Labradorite/spectrolite (semi-precious)	Plagioclase feldspar (An₅₀–₇₀)	(Ca,Na)(Al,Si)₄O₈	Gray body; labradorescence from light interference in exsolution lamellae	6–6.5	—	Spectrolite = dark, full-spectrum labradorite from Ylämaa, Finland; Labrador, Canada; Madagascar
Amazonite (semi-precious)	Microcline (K-feldspar)	KAlSi₃O₈	Green to blue-green: trace Pb plus structural water, activated by natural irradiation	6–6.5	—	Two 90° cleavages make it fragile for rings; Pikes Peak, Colorado; Russia; Madagascar
Turquoise (semi-precious)	Turquoise (phosphate)	CuAl₆(PO₄)₄(OH)₈·4H₂O	Sky blue: Cu²⁺ (idiochromatic); greener with Fe³⁺ replacing Al	5–6	December	Porous, often stabilized; dark veining = matrix; Nishapur, Iran; Sinai, Egypt; Arizona, Nevada, New Mexico
Topaz (semi-precious)	Topaz	Al₂SiO₄(F,OH)₂	Imperial (golden-orange to pink): Cr³⁺ plus color centers; sherry/brown: color centers; blue: usually irradiated and heated colorless topaz	8	November	Perfect basal cleavage; imperial from Ouro Preto, Brazil
Tourmaline: rubellite, indicolite, watermelon (semi-precious)	Tourmaline (mostly elbaite)	e.g. elbaite Na(Li₁.₅Al₁.₅)Al₆(Si₆O₁₈)(BO₃)₃(OH)₄	Rubellite red-pink: Mn³⁺ (often from natural irradiation of Mn²⁺); indicolite blue: Fe²⁺ (with Fe²⁺–Fe³⁺ charge transfer); watermelon: pink core (Mn) and green rind (Fe²⁺) from changing fluid chemistry during growth; Paraíba neon blue: Cu²⁺	7–7.5	October	Strong pleochroism; piezo- and pyroelectric; Minas Gerais and Paraíba, Brazil; Pala, California; Maine
Garnet: almandine, tsavorite (grossular) (semi-precious)	Garnet group	Almandine Fe₃Al₂(SiO₄)₃; grossular Ca₃Al₂(SiO₄)₃	Almandine deep red: Fe²⁺ (idiochromatic); tsavorite green: V³⁺ and Cr³⁺	6.5–7.5	January	Singly refractive (isometric); tsavorite from Tsavo, Kenya and Merelani, Tanzania
Zircon (semi-precious)	Zircon	ZrSiO₄	Natural brown/red: trace elements and radiation damage; blue, golden and colorless gems usually heat-treated	6–7.5	December	High dispersion (fire) and doubled facets; weakly radioactive; not cubic zirconia (synthetic ZrO₂); Ratanakiri, Cambodia; Sri Lanka
Diamond (precious)	Diamond (native carbon)	C	Colorless when pure; yellow: nitrogen; blue: boron; pink/brown: plastic deformation of the lattice; green: natural radiation	10	April	Adamantine luster, highest dispersion of common gems (0.044); kimberlite and lamproite pipes, placers; South Africa, Botswana, Russia, Canada
Spodumene: kunzite, hiddenite (semi-precious)	Spodumene	LiAlSi₂O₆	Kunzite pink-lilac: Mn²⁺/Mn³⁺ (fades in sunlight); hiddenite green: Cr³⁺	6.5–7	—	Two perfect cleavages make cutting difficult; strong pleochroism; kunzite fluoresces orange-pink; named after G. F. Kunz and W. E. Hidden; Pala, California; Hiddenite, North Carolina; Afghanistan
Pearl (organic gem; traditionally sometimes counted precious)	Not a mineral: aragonite (CaCO₃) tablets + conchiolin protein, made by molluscs	CaCO₃ + organic matter	White, cream, pink, gold, black: body color from organic pigments and host; lustre and iridescent "orient" from light interference in layered nacre	2.5–4.5	June	Biogenic, so not a mineral; fizzes in acid (carbonate); feels gritty on teeth; natural vs cultured (bead nucleus)
Amber (organic gem; semi-precious)	Not a mineral: fossil tree resin	Variable organic (approx. C₁₀H₁₆O)	Yellow, orange, brown (rarely blue-fluorescing, Dominican): organic compounds in the resin	2–2.5	—	Amorphous and organic; SG ~1.05–1.10 (floats in saturated salt water); static charge when rubbed (Greek elektron); fossil inclusions; Baltic coast (succinite); Dominican Republic; Myanmar

Mineral look-alike pairs

ReferenceMineral look-alike pairs· 61 rows

The single test that separates each confusable pair or set, with the values to quote. Grouped roughly by family; the silicate and gem rows follow the non-silicates.

61 rows

Pair	Decisive test with values
Gold vs pyrite vs chalcopyrite	Gold: golden-yellow streak, SG 19.3, malleable (dents). Pyrite (H 6–6.5) and chalcopyrite (H 3.5–4) give greenish-black streaks and crumble when pressed
Pyrite vs chalcopyrite	Pyrite H 6–6.5 (a knife cannot scratch it), pale brass, striated cubes; chalcopyrite H 3.5–4 (knife scratches it), deeper greenish-yellow, iridescent tarnish
Bornite vs tarnished chalcopyrite	Scratch to a fresh surface: coppery red to bronze = bornite (streak grayish black, H 3); brass-yellow = chalcopyrite (streak greenish black, H 3.5–4)
Galena vs stibnite	Both lead-gray with lead-gray streak. Galena: cubic cleavage (3 at 90°), SG 7.5. Stibnite: one lengthwise cleavage, bladed striated crystals, SG 4.6
Sphalerite vs galena	Sphalerite: pale yellow-brown streak, resinous luster, 6 cleavages, SG about 4. Galena: lead-gray streak, metallic, SG 7.5
Graphite vs other black metallics	Graphite: H 1–2, SG about 2.2, greasy, marks paper. Galena 7.5 and magnetite 5.2 are far heavier
Hematite vs magnetite vs goethite (all can be black)	Streak: red-brown = hematite; black and strongly magnetic = magnetite; yellow-brown = goethite/limonite. Hematite is not attracted by an ordinary magnet
Goethite vs limonite	Same yellow-brown streak. Goethite is crystalline (orthorhombic, fibrous, submetallic); limonite is an amorphous, earthy mixture. One answer on the list
Pyrolusite vs magnetite vs graphite	Pyrolusite soils fingers, black streak, not magnetic; magnetite strongly magnetic, H 5.5–6.5; graphite greasy and light (SG 2.2)
Rutile vs hematite	Rutile: pale brown streak, striated prisms or needles, SG 4.2. Hematite: red-brown streak, SG 5.3
Zincite vs red sphalerite	Zincite: orange-yellow streak, one basal cleavage, almost only from Franklin, NJ. Sphalerite: white to pale yellow streak, 6 cleavages
Silver vs copper	Fresh color and streak: silver-white (black tarnish) vs copper-red (green or brown tarnish); in nitric acid only copper gives a blue-green solution
Diamond vs graphite	Same formula (C), different structure (polymorphs): diamond H 10, SG 3.52, adamantine; graphite H 1–2, SG 2.2, metallic and greasy
Halite vs fluorite	Halite: cubic cleavage (3 at 90°), H 2.5, salty, dissolves in water. Fluorite: octahedral cleavage (4), H 4, insoluble
Halite vs calcite (clear cleavage pieces)	Halite cleaves into 90° cubes, no double image. Calcite cleaves into rhombs (75°/105°), fizzes, shows strong double refraction
Fluorite vs amethyst (purple)	Fluorite H 4 (knife scratches it), 4 cleavages. Amethyst H 7, no cleavage, conchoidal fracture
Calcite vs dolomite vs magnesite, siderite, rhodochrosite	Acid: calcite fizzes in cold dilute HCl; dolomite only powdered or warm; magnesite, siderite, rhodochrosite only in warm acid. SG: calcite 2.71, dolomite 2.85, magnesite 3.0, rhodochrosite 3.6, siderite 3.9
Calcite vs aragonite	Polymorphs. Calcite: 3 rhombohedral cleavages, SG 2.71. Aragonite: poor cleavage, needles or columns, pseudo-hexagonal twins, SG 2.95
Azurite vs malachite	Both fizz. Azurite: deep blue, light-blue streak. Malachite: banded green, light-green streak
Ulexite vs satin spar vs fibrous calcite	All white and fibrous. Ulexite transmits an image ("TV rock"); satin spar is H 2 and does not fizz; fibrous calcite fizzes
Rhodochrosite vs rhodonite	Rhodochrosite: H 3.5–4, rhombohedral cleavage, white banding, fizzes in warm acid. Rhodonite: H 5.5–6.5, black Mn-oxide veins, no fizz
Barite vs celestite	Same structure. Barite SG about 4.5, green flame (Ba). Celestite SG about 4.0, crimson flame (Sr), often pale blue
Barite vs calcite	Barite is far heavier (4.5 vs 2.71) and does not fizz
Gypsum vs calcite vs halite	Gypsum H 2 (fingernail scratches it), no fizz, not salty; calcite H 3, fizzes; halite H 2.5, salty, cubic cleavage
Selenite vs muscovite	Both peel. Selenite sheets are flexible but stay bent and are H 2; muscovite sheets spring back (elastic)
Apatite vs beryl	Both hexagonal prisms. Apatite H 5 (knife scratches it); beryl H 7.5–8
Pyromorphite vs vanadinite	Same structure and habit, both SG about 7. Pyromorphite green to yellow; vanadinite red-orange
Turquoise vs chrysocolla	Turquoise H 5–6, SG 2.6–2.9; chrysocolla H 2–4, SG 1.9–2.4, sticks to the tongue
Hornblende vs augite	Hornblende: cleavages at about 56°/124°, elongate diamond-shaped sections. Augite: about 87°/93°, stubby nearly square sections. Both dark, H 5–6
Actinolite vs tremolite vs epidote	Tremolite white or gray (no Fe), actinolite green; both have 56°/124° amphibole cleavage. Epidote pistachio-green, one perfect cleavage, lengthwise striations, H 6–7
Epidote vs olivine	Epidote: pistachio (yellow-green) color, one perfect cleavage, prismatic crystals striated lengthwise; in metamorphic rocks and quartz veins. Olivine: olive-green granular grains, no cleavage (conchoidal fracture); in mafic igneous rocks (basalt, gabbro, peridotite)
Olivine vs epidote vs green grossular	Olivine granular, no good cleavage, in basalt; epidote pistachio, one perfect cleavage, striated prisms; grossular forms dodecahedra (H 6.5–7.5, SG 3.4–3.7)
Plagioclase (albite, labradorite) vs K-feldspar	Straight, parallel twin striations on one cleavage face = plagioclase (labradorite also flashes blue-green). Carlsbad twins, wavy white perthite and pink or tan color = orthoclase/microcline; amazonite is green K-feldspar
Green set: amazonite, dioptase, emerald, chrysocolla	Amazonite: 2 cleavages at 90°, white perthite, H 6–6.5. Dioptase: H 5, green streak, rhombohedral cleavage. Emerald: H 7.5–8, no good cleavage. Chrysocolla: H 2–4, earthy, sticks to the tongue
Almandine vs grossular	Color (deep red vs green or orange) and SG (4.1–4.3 vs 3.4–3.7)
Beryl vs quartz vs apatite	Beryl: hexagonal prism with a flat top, H 7.5–8. Quartz: pointed terminations, H 7. Apatite: H 5 (knife scratches it)
Kaolinite vs chalk vs talc	Chalk fizzes in HCl; talc is soapy, H 1; kaolinite has an earthy smell, is plastic when wet, H 2–2.5
Kyanite vs sillimanite vs andalusite (all Al₂SiO₅)	Kyanite: blue blades with directional hardness (high P). Sillimanite: white fibrous needles (fibrolite), H 6.5–7.5 all round (high T). Andalusite: square pinkish prisms, chiastolite cross (low P)
Biotite vs muscovite vs lepidolite vs chlorite	All split into sheets. Biotite black or brown, muscovite colorless or silvery, lepidolite lilac (crimson Li flame); all mica flakes are elastic. Chlorite is green and its flakes bend but do not spring back
Talc vs gypsum (alabaster) vs muscovite	Talc H 1 and soapy, flakes stay bent; gypsum H 2, not soapy; muscovite flakes are elastic
Chlorite vs serpentine vs talc	Chlorite: green flakes in foliated rocks. Serpentine: massive or fibrous, waxy, mottled green, H 2.5–4. Talc: white to pale green, H 1, soapy
Augite vs spodumene	Same pyroxene cleavage (about 87°/93°). Augite dark and heavier (SG 3.2–3.6); spodumene pale gray, pink or green in pegmatite (SG 3.0–3.2)
Sodalite vs lapis lazuli vs azurite	Sodalite: white streak, SG 2.3, no pyrite. Lapis: blue streak with pyrite flecks. Azurite: light-blue streak, SG 3.8, fizzes
Staurolite vs garnet	Staurolite: brown elongate prisms and cross-shaped twins. Garnet: equant red dodecahedra
Stilbite vs apophyllite	Stilbite: sheaves of pearly tabular crystals, H 3.5–4. Apophyllite: square pseudo-cubic prisms with pearly "fish-eye" tops, H 4.5–5
Topaz vs quartz (citrine)	Topaz H 8, one perfect basal cleavage, SG 3.5; quartz H 7, no cleavage, SG 2.65
Black tourmaline (schorl) vs hornblende	Schorl: rounded triangular cross-section, deep lengthwise striations, no good cleavage, H 7–7.5. Hornblende: 56°/124° cleavage, H 5–6
Willemite vs calcite (Franklin)	Willemite glows bright green under SW UV, H 5.5; calcite glows red there, H 3, fizzes
Wollastonite vs tremolite	Wollastonite: splintery white blades in skarn, H 4.5–5, no 56° cleavage. Tremolite: amphibole cleavage, H 5–6
Zircon vs diamond vs cubic zirconia	Diamond H 10, singly refractive. Zircon H 7.5, doubly refractive (doubled back facets). Cubic zirconia H about 8, very heavy (SG about 5.6–6), synthetic
Chrysotile vs tremolite asbestos vs satin spar	Chrysotile is serpentine (soft, silky, in serpentinite); tremolite asbestos is an amphibole in marble or talc deposits; satin spar is gypsum (H 2, in evaporites)
Chalcedony vs jasper vs opal	Chalcedony translucent and waxy, H 6.5–7; jasper opaque, dull, colored by iron oxides; opal softer (H 5.5–6.5), lighter (SG about 2.1), may show play of color
Citrine vs topaz	Citrine H 7, no cleavage, SG 2.65; topaz H 8, perfect basal cleavage, SG 3.5. Heated citrine is redder-orange with darker tips; natural citrine is pale and even
Smoky quartz vs obsidian	Smoky quartz is crystalline with prism faces, transparent on thin edges; obsidian is volcanic glass with no faces (a rock)
Aventurine vs amazonite	Aventurine glitters and has no cleavage (H 7); amazonite has two 90° cleavages and white perthite streaks
Tiger's eye vs satin spar	Both silky and fibrous; tiger's eye H 7, satin spar H 2 (fingernail scratches it)
Rock crystal vs calcite	Quartz H 7, no cleavage, no fizz; calcite H 3, rhombohedral cleavage, strong double refraction, fizzes
Ruby vs red garnet	Ruby H 9, SG about 4.0, doubly refractive, fluoresces red; almandine H 7–7.5, SG 4.1–4.3, singly refractive, no fluorescence
Emerald vs green glass or dioptase	Emerald H 7.5–8 with natural inclusions; dioptase H 5 with cleavage; glass shows round bubbles and is H about 5.5
Pearl vs imitation	Real pearl feels gritty against teeth and fizzes in acid; glass or plastic beads feel smooth
Amber vs copal vs plastic	Amber floats in saturated salt water and resists a drop of solvent better than copal; most plastics sink

Rock reference tables

One property line per rock on the National List, grouped by rock family, then the look-alike pairs that cost the most points. Rocks marked * are State and National only, and "not on list" rows are comparison rocks that the rules name in the facies or settings topics. Use the filter box to search by name, texture, mineral or environment.

Igneous rock table

ReferenceIgneous rocks: reference table· 14 rows

• = State/National only. SiO₂ = weight % silica (TAS/IUGS boundaries; textbooks round them). CI = color index (volume % dark minerals) on the guide's working scale: felsic ≈0–15, intermediate ≈15–45, mafic ≈45–85, ultramafic >85. Plag = plagioclase; K-spar = potassium feldspar. Dacite, trachyte, granodiorite and komatiite appear only as comparison names: not on the ID list. The texture × composition grid is in Naming a rock: the classification grid.

14 rows

Rock	Texture	Composition (felsic–ultramafic) & essential minerals	Silica %	Color index / typical color	Intrusive or extrusive; body	Plate setting / where it forms	Extrusive ↔ intrusive equivalent	Look-alikes & how to tell	Uses
Andesite	Aphanitic; very often porphyritic (white plag, black hornblende or pyroxene phenocrysts); may be vesicular or flow-banded	Intermediate; plag (andesine) + hornblende and/or pyroxene ± biotite; little or no quartz	≈57–63 (basaltic andesite 52–57)	CI ≈15–45; medium to dark gray, also greenish, purplish or brown	Extrusive; lava flows, lava domes, layers in stratovolcanoes (composite cones)	Subduction-zone volcanic arcs, ocean–continent and ocean–ocean (Andes, Cascades, Japan)	Diorite	Basalt is darker, denser, phenocrysts (if any) are olivine/plag not hornblende; rhyolite is lighter (pink, tan) and may show quartz phenocrysts; supervisor may give mineral makeup	Crushed stone, road aggregate, local building stone
Basalt	Aphanitic; commonly vesicular or amygdaloidal; may be porphyritic (olivine, plag); pillows and columnar joints in outcrop	Mafic; Ca-rich plag (labradorite) + augite ± olivine, magnetite	≈45–52	CI ≈45–85; dark gray to black, weathers rusty brown	Extrusive; lava flows, pillow lavas, flood basalts, shield volcanoes; shallow dikes and sills (diabase/dolerite)	Mid-ocean ridges (upper oceanic crust), hot spots (Hawaii, Iceland), continental rifts, flood-basalt provinces (Columbia River, Deccan)	Gabbro	Gabbro has visible crystals; andesite is lighter gray with hornblende phenocrysts; dark chert and hornfels are harder (H ≈7) with no vesicles; slate splits into sheets	Crushed stone, aggregate, railroad ballast, rock (mineral) wool insulation, basalt fiber
Diorite	Phaneritic, medium to coarse; "salt-and-pepper" speckling; sometimes porphyritic	Intermediate; Na-rich plag (white) + hornblende ± biotite, pyroxene; quartz under ≈5% (more = quartz diorite)	≈52–63	CI ≈15–45; roughly equal black and white grains	Intrusive; plutons, stocks, parts of batholiths, dikes	Subduction-zone continental arcs, beneath andesite volcanoes	Andesite	Granite has glassy gray quartz and often pink K-spar, fewer dark grains; gabbro is darker (mostly dark minerals, pyroxene not hornblende, plag gray not white)	Dimension and building stone, crushed stone; carved by ancient cultures for statues and vessels
Gabbro	Phaneritic, coarse; sometimes layered (cumulate)	Mafic; Ca-rich plag (gray, labradorite) + augite ± olivine, hornblende	≈45–52	CI ≈45–85; dark gray to black, greenish	Intrusive; layered intrusions (Bushveld, Stillwater, Duluth), sills, lower oceanic crust (layer 3)	Mid-ocean ridges (cools beneath them), rifts, hot spots; exposed in ophiolites	Basalt	Diorite is lighter with white plag and hornblende; amphibolite has aligned shiny hornblende and is often foliated; peridotite has abundant green olivine and almost no plag	Sold as "black granite" for countertops, monuments; aggregate; hosts Ni–Cu–platinum-group ores
Granite	Phaneritic, medium to coarse; sometimes porphyritic (large K-spar)	Felsic; quartz (≈20–60% of light minerals) + K-spar (orthoclase/microcline) + Na-plag ± biotite, muscovite, hornblende; feldspars usually over half the rock, quartz commonly a quarter to a third, dark minerals under ≈15%	≈70–77 (felsic class above ≈65)	CI ≈0–15; pink, white, light gray, speckled	Intrusive; batholiths and stocks (Sierra Nevada, Half Dome, Stone Mountain)	Continental convergent margins (arcs), continental collision zones; continental crust only	Rhyolite	Syenite has little or no quartz; diorite has more dark minerals and little or no quartz; gneiss is banded; arkose has cemented angular-to-subangular grains with pores and breaks around them; pegmatite is much coarser	Dimension and building stone, countertops, monuments and gravestones, curling stones, crushed stone
Obsidian	Glassy (no crystals); conchoidal fracture with sharp edges; may hold spherulites or flow bands	Usually felsic (rhyolitic) volcanic glass; amorphous, a mineraloid rock; very low water	≈70–77	Color does not reflect composition: black (tiny magnetite), brown or "mahogany" (hematite), snowflake (white cristobalite spherulites), sheen/rainbow	Extrusive; edges and tops of rhyolite lava flows and domes (Obsidian Cliff, Yellowstone)	Continental arcs, continental hot spots, rifts	Same chemistry as rhyolite/granite (glassy form)	Dark chert/flint is opaque, waxy-dull, H ≈7; obsidian is glassy, translucent on thin edges, H ≈5–5.5; slag glass usually has bubbles and bright colors	Blades, arrowheads and scrapers; specialty surgical scalpels (limited use); ornaments and carvings
Pegmatite	Pegmatitic: very coarse, crystals ≥1 cm to meters; graphic intergrowths of quartz in K-spar	Usually felsic (granitic); quartz + K-spar (microcline) + albite + muscovite ± tourmaline, beryl, spodumene, lepidolite, garnet, topaz	≈70+ (granitic)	CI very low; white, pink, gray with black tourmaline or biotite books	Intrusive; dikes, veins, lenses and pods near batholith margins; slow cooling of the last, water- and volatile-rich melt (fluids speed ion diffusion, so few crystals grow huge)	Continental orogenic belts around granite batholiths	— (texture comes from fluid-rich late melt, not from a lava)	Granite has crystals mostly under ≈1 cm and evenly sized; gneiss is banded	Ores of Li (spodumene, lepidolite), Be (beryl), Cs, Ta, Nb, Sn; gemstones (tourmaline, beryl, topaz); feldspar and sheet mica; high-purity quartz
Peridotite	Phaneritic, coarse; granular	Ultramafic; olivine (≥40%) + ortho- and clinopyroxene ± garnet, spinel, chromite; dunite = >90% olivine	<45 (≈40–45)	CI >85 (usually >90); olive to dark green, weathers brown; dense (SG ≈3.2–3.3)	Intrusive/mantle; upper mantle, bases of layered intrusions, mantle slices in ophiolites, xenoliths in basalt and kimberlite	Makes up most of the upper mantle; exposed where ophiolites are thrust up at convergent boundaries	Komatiite (rare, almost all Archean; not on the ID list)	Gabbro has much plag and less olivine; serpentinite is waxy, softer and mottled; dunite is the olivine-only variety	Chromite, nickel, platinum-group metals; peridot gems; olivine foundry sand and refractories; CO₂ mineral capture research
Pumice	Vesicular and glassy; frothy with many small, often elongated (tube) vesicles and thin glass walls; porosity ≈64–85% (some sources say up to ≈90%)	Felsic to intermediate (rhyolitic to dacitic; dacite is not on the ID list) glass	≈65–75	CI very low; white, cream, light gray	Extrusive; pumice falls and pyroclastic flows from explosive eruptions; frothy tops of silica-rich flows	Continental arcs and calderas (explosive, gas-rich felsic magma)	Same chemistry as rhyolite/granite (frothy glass form)	Scoria is dark, heavier, larger round vesicles and sinks; pumice is light-colored and usually floats; diatomite is powdery with no vesicles	Abrasive (pumice stones, polishes, stonewashed jeans), lightweight concrete aggregate and blocks, horticulture, pozzolan
Rhyolite	Aphanitic; often porphyritic (quartz, sanidine phenocrysts), flow-banded, may be partly glassy or spherulitic	Felsic; quartz + K-spar (sanidine) + Na-plag ± biotite	≈69–77	CI ≈0–15; pink, tan, light gray, lavender, reddish	Extrusive; viscous lava domes, thick short flows, ignimbrites from caldera eruptions	Continental arcs, continental hot spots (Yellowstone), continental rifts	Granite	Andesite is darker gray with hornblende phenocrysts; chert is waxy, H 7, no phenocrysts; tuff has visible ash or pumice fragments	Crushed stone, aggregate; prehistoric tool stone; ornamental (wonderstone, spherulitic varieties)
Scoria	Vesicular; vesicles larger and rounder than in pumice with thicker walls; glassy to microcrystalline	Mafic to intermediate (basaltic, andesitic); glass + small plag, pyroxene, olivine	≈45–57	CI high; black, dark gray, reddish-brown (oxidized iron)	Extrusive; cinder (scoria) cones, rubbly tops of lava flows, Strombolian fountains	Hot spots, rifts and arcs; anywhere basaltic eruptions are gas-rich	Vesicular form of basalt chemistry (intrusive equivalent gabbro)	Pumice is pale, floats, has fine glassy froth; vesicular basalt has fewer holes (mostly solid rock)	Landscaping "lava rock", lightweight aggregate, grill briquettes, drainage and road traction
Syenite*	Phaneritic, medium to coarse; sometimes trachytoid (aligned feldspar laths)	Intermediate (alkaline); K-spar dominant ± Na-plag, hornblende, biotite, aegirine; quartz under ≈5%; feldspathoids in nepheline syenite	≈55–65	CI ≈5–30 (paler than most intermediate rocks because its feldspar is pink or white K-spar); pink, gray, light colors	Intrusive; small plutons, dikes, ring complexes	Continental rifts and intraplate (hot spot) alkaline provinces	Trachyte (not on the ID list)	Granite has ≥20% quartz (gray glassy grains); diorite is plag-dominant and darker; supervisor gives quartz content	Dimension stone (e.g. larvikite, a monzonite–syenite "blue pearl granite"); nepheline syenite for glass and ceramics
Tuff*	Pyroclastic; mainly ash (<2 mm) with glass shards, crystals, lithic and pumice fragments; welded (ignimbrite) or unwelded	Usually felsic to intermediate (rhyolitic to dacitic) ash; mafic tuffs also exist	Variable; commonly ≈60–75	Low; white, cream, buff, pink, pale gray; welded tuff darker	Extrusive; ash-fall layers and pyroclastic-flow sheets (ignimbrites); subaerial, submarine, lacustrine or reworked	Explosive volcanoes at subduction arcs and calderas	— (fragmental; no intrusive equivalent)	Sandstone has rounded mineral grains, no glass shards or pumice, is heavier; pumice is one frothy piece; tuff breccia has many blocks over 64 mm	Building stone (Rome, Cappadocia, Easter Island moai); pozzolan for Roman concrete; zeolite source
Tuff breccia*	Pyroclastic; 25–75% angular blocks/bombs >64 mm in an ash (tuff) matrix; poorly sorted	Usually intermediate to felsic volcanic fragments; glass, crystals, lava fragments	Variable (that of the fragments)	Mottled, multicoloured fragments in pale matrix	Extrusive; near-vent pyroclastic flows, block-and-ash flows, vent and collapse breccias	Explosive volcanoes at subduction arcs	— (fragmental)	Sedimentary breccia has non-volcanic clasts and no ash/glass matrix; tuff is mostly ash; conglomerate clasts are rounded	Local building stone, aggregate

Sedimentary rock table

ReferenceSedimentary rocks: reference table· 23 rows

• = State/National only. Wentworth: clay <1/256 mm (0.004), silt 1/256–1/16 mm (0.004–0.0625), sand 1/16–2 mm, granule 2–4, pebble 4–64, cobble 64–256, boulder >256 mm. "Fizz" = reaction with cold dilute HCl. Coal carbon % = USGS rank ranges. Rock salt and rock gypsum are identified the same as the minerals halite and gypsum.

23 rows

Rock	Texture class (clastic / chemical / biochemical)	Grain size (Wentworth) / sorting / rounding	Composition & cement	Depositional environment & energy	Diagnostic features (fizz, fossils, feel, structures)	Metamorphoses to	Look-alikes & how to tell	Uses
Banded iron formation	Chemical (iron precipitated as microbes oxygenated the sea)	Microcrystalline; mm- to cm-thick bands; no clasts	Alternating iron oxide (hematite, magnetite) and chert or jasper (SiO₂) bands	Marine shelves and basins, low energy; dissolved Fe²⁺ from hydrothermal vents oxidized by O₂ from cyanobacteria; mostly 2.6–1.8 Ga (oldest about 3.8 Ga; a brief return about 0.75 Ga), peaking around the Great Oxidation Event (≈2.4 Ga)	Red/black/silver striped layers; heavy; magnetic where magnetite-rich; no fizz; H ≈5.5–7	Metamorphosed iron formation (itabirite, magnetite quartzite)	Jasper or chert is one color and light; BIF has metallic oxide bands and is heavy; "tiger iron" is BIF with tiger's eye	World's main iron ore (taconite pellets from the Mesabi Range, Minnesota); ornamental (tiger iron)
Bauxite	Chemical (residual weathering product; grouped with sedimentary rocks on the rock list)	Earthy, clay-size matrix with pisolites/ooids (round concretions, often >2 mm); unsorted, not transported	Al hydroxides: gibbsite Al(OH)₃, boehmite and diaspore AlO(OH); plus kaolinite, goethite, hematite	Formed in place on land by intense chemical weathering (laterization) of aluminous rock in hot, wet, well-drained tropical climates	Earthy, reddish-brown, tan or cream; pisolitic; soft (H ≈1–3); light; no fizz	Diaspore–corundum rock (emery, e.g. Naxos)	Pisolitic limestone fizzes; laterite is iron-dominant (darker, heavier); clay lacks pisolites	Main ore of aluminum (Bayer process: bauxite → alumina Al₂O₃; then Hall–Héroult electrolysis of alumina dissolved in molten cryolite → aluminum metal); refractories, abrasives
Breccia	Clastic	Gravel (>2 mm), angular clasts; poorly sorted; little transport	Rock fragments of any kind in sand/mud matrix; cement calcite, silica or iron oxide	High energy, near source: talus/scree, alluvial fans, landslides, fault zones, cave collapse, impacts	Angular broken clasts; mixed sizes	Metabreccia (clasts flattened in gneiss or schist)	Conglomerate clasts are rounded; tuff breccia has volcanic blocks in ash	Decorative stone, aggregate
Chert/Flint	Chemical or biochemical (siliceous)	Microcrystalline to cryptocrystalline; no visible grains	Microcrystalline quartz/chalcedony (SiO₂); flint = dark gray–black chert nodules in chalk	Deep-sea siliceous oozes (radiolarians, diatoms) recrystallized; nodules replacing limestone or chalk; low energy	H 7 (scratches glass); conchoidal fracture with sharp edges; waxy to dull luster; no fizz; jasper = red chert	Quartzite (metachert)	Obsidian is glassy, H ≈5.5; jasper is red chert; quartzite shows sugary grains	Stone tools and weapons, fire starting with steel, gunflints, flint building walls, abrasives
Conglomerate	Clastic	Gravel (>2 mm), rounded clasts; poorly to moderately sorted	Rounded pebbles (often quartz, chert, rock fragments) in sand matrix; cement silica, calcite, iron oxide	High energy: river channels, alluvial fans, gravel beaches, glacial outwash	Rounded pebbles in finer matrix; may show crude bedding	Metaconglomerate (stretched-pebble)	Breccia clasts are angular; tillite is unsorted with striated clasts	Decorative stone, aggregate; host of gold and uranium ores (Witwatersrand)
Diatomite*	Biochemical (siliceous)	Silt to clay size; microscopic diatom frustules; very porous	Opal-A (amorphous SiO₂·nH₂O) diatom skeletons ± clay	Low-energy marine or lake water rich in nutrients (diatom blooms)	Very light (bulk density far below 1 g/cm³ when dry); white/cream, chalky, powdery; white streak; no fizz	Diagenesis → porcelanite and chert; metamorphism → quartzite	Chalk fizzes and is heavier; pumice is vesicular glass; kaolin clay is plastic when wet	Filter aid (pools, beer, wine, water), mild abrasive (polishes, toothpaste), absorbent (cat litter; Nobel's dynamite), insulation, natural insecticide, filler
Dolostone	Chemical (mostly by dolomitization: Mg-rich water replaces limestone)	Crystalline, fine to coarse; often sugary (sucrosic); fossils may be blurred	Dolomite CaMg(CO₃)₂ (>50%) ± calcite	Shallow warm marine, tidal flats, sabkhas, evaporitic lagoons; low energy	Fizzes only weakly or when powdered or in warm acid; weathers buff/tan; H ≈3.5–4; porous	Dolomitic marble	Limestone fizzes vigorously on the bare surface	Crushed stone, building stone, dolomitic agricultural lime, Mg source, steelmaking flux; oil reservoirs; host of lead–zinc ores
Rock salt (halite)	Chemical (evaporite)	Crystalline, medium to coarse	Halite NaCl ± gypsum, anhydrite, sylvite	Evaporation of seawater in restricted basins, sabkhas, salt lakes/playas; arid, low energy	Cubic cleavage; H 2.5; salty taste (no taste test in the event); dissolves in water; colorless, white, pink, orange	Does not metamorphose readily; flows plastically into salt domes	Rock gypsum is softer (H 2), no cubic cleavage, only very slightly soluble; calcite fizzes, rhombohedral cleavage	Table salt, road de-icing, chlorine and NaOH industry, food preservation; salt caverns store oil and gas
Rock gypsum	Chemical (evaporite; first major evaporite after carbonate)	Crystalline, fine to coarse; massive (alabaster), fibrous (satin spar) or crystals (selenite)	Gypsum CaSO₄·2H₂O ± anhydrite	Evaporitic basins, sabkhas, playas; arid, low energy	H 2 (fingernail scratches); no fizz; white, gray, pink; sugary to fibrous	Dehydrates to anhydrite on burial	Rock salt has cubic cleavage, H 2.5, dissolves; marble and alabaster-like calcite fizz, H 3	Plaster of Paris, drywall (wallboard), cement retarder, soil conditioner, carving (alabaster)
Shale	Clastic	Clay and silt (<1/16 mm, mostly <1/256 mm); well sorted	Clay minerals (illite, kaolinite, smectite) + quartz, organic matter; compacted, little cement	Low energy: deep marine, lakes, lagoons, floodplains, offshore deltas	Fissile (splits into thin layers parallel to bedding); smooth feel; soft; earthy smell when breathed on; may hold fossils; black shale = organic-rich	Slate → phyllite → schist → gneiss	Mudstone is not fissile; siltstone feels gritty; slate is harder, rings when tapped, cleavage may cut bedding	Bricks, tiles, cement raw material; oil shale; source rock and reservoir for shale gas and oil
Siltstone*	Clastic	Silt (1/256–1/16 mm); grains barely visible with a lens; well sorted	Quartz, feldspar, clay; cement silica, calcite or iron oxide	Low to moderate energy: floodplains, deltas, lakes, quiet shelf, loess	Slightly gritty (not smooth like shale); not usually fissile; thin laminations, ripple marks	Slate or phyllite (argillite at very low grade)	Shale is smooth and fissile; very fine sandstone shows grains with the naked eye	Flagstone, building stone
Peat (not on list)	Biochemical (organic)	Unconsolidated plant fibers; not a lithified rock	Partly decayed plant matter; carbon below ≈60% (dry); very high moisture	Waterlogged bogs, swamps, marshes; low oxygen	Brown, spongy, visible roots and moss; lightweight; burns smoky	Ranks to lignite with burial	Lignite is compact and rock-like	Fuel (Ireland, Scotland), horticulture
Anthracite	Biochemical (organic) coal; often classed metamorphic (low-grade metamorphosed coal)	Non-clastic; dense, glassy	Carbon ≈86–97%; very low volatiles and moisture	Peat-swamp coal buried deeply and heated, usually in folded mountain belts (eastern Pennsylvania)	Black, submetallic to vitreous luster; conchoidal fracture; hard for coal; does not soil fingers; burns hot with little smoke	Graphite	Bituminous is banded, dull and bright, blocky, soils fingers; obsidian is heavier and translucent on edges; jet is lighter	Home heating, metallurgy, water-filter media; highest energy per kg of the coals
Bituminous	Biochemical (organic) coal	Non-clastic; banded bright (vitrain) and dull layers; blocky, cleats	Carbon ≈45–86%	Peat-swamp coal under moderate burial and heat	Black, banded bright/dull; blocky; soils fingers; burns with smoky flame	Anthracite	Anthracite is uniformly shiny, conchoidal, clean; lignite is brown and crumbly	Electricity generation; coking (metallurgical) coal for steel; most abundant US coal rank
Lignite	Biochemical (organic) coal	Non-clastic; soft, earthy to woody; plant structures may be visible	Carbon ≈25–35%; high moisture	Shallowly buried peat-swamp deposits; geologically young	Brown to brownish-black; crumbles as it dries; light; lowest-rank coal	Sub-bituminous, then bituminous coal	Bituminous is black and harder; peat is unconsolidated	Electricity at mine-mouth power plants; lowest energy per kg
Chalk	Biochemical (calcareous)	Clay to silt size; coccoliths and foraminifera; very porous	Calcite CaCO₃ microfossils	Warm, quiet marine shelf seas below wave base (Cretaceous; White Cliffs of Dover)	White, soft, powdery; writes on surfaces; white streak; light; fizzes vigorously; may hold flint nodules	Marble	Diatomite does not fizz and is even lighter; kaolin clay does not fizz	Cement and lime, agricultural lime, filler (paper, paint, putty); writing chalk (much modern blackboard chalk is gypsum)
Coquina	Biochemical (bioclastic)	Sand to gravel size whole and broken shells; poorly cemented; very porous	Calcite/aragonite shells	High-energy beaches, shoals and surf zone	Loosely cemented, obvious shell fragments; porous; fizzes	Marble	Fossil limestone has fossils set in dense fine matrix and is well cemented	Building stone (Castillo de San Marcos, St. Augustine), road base, agricultural lime
Fossil limestone	Biochemical	Whole fossils (brachiopods, crinoids, corals, bryozoans) in fine micrite or sparry calcite	Calcite, well cemented	Shallow warm marine shelves and reefs; low to moderate energy	Fizzes; visible fossils in dense matrix	Marble (fossils destroyed)	Coquina is mostly loose shell hash with high porosity; marble has no fossils, sparkly cleavage faces	Building and dimension stone, cement and lime, aggregate, decorative stone
Oolitic limestone	Chemical (inorganic precipitation; some texts biochemical)	Sand-size ooids (≈0.25–2 mm), spherical with concentric layers around a nucleus; well sorted	Calcite (ooids often originally aragonite); sparry calcite cement	Warm, shallow, constantly agitated (high energy) water supersaturated with CaCO₃: tidal shoals, bars (Bahamas)	Looks like fish roe; fizzes	Marble	Pisolitic limestone has pisoids >2 mm; bauxite pisolites do not fizz; sandstone grains are not concentric and do not fizz	Building stone (e.g. Bath stone, Portland stone), cement and lime
Travertine	Chemical	Crystalline, banded, porous with vugs; fibrous crusts	Calcite or aragonite	Hot and cold springs (CO₂ degassing: Mammoth Hot Springs, Pamukkale) and caves (flowstone, dripstone); terrestrial	Concentric or wavy bands; holes; cream, tan, red; fizzes; tufa is a softer, more porous cold-water form; porosity drops with age as later calcite precipitates inside the pores	Marble	Oolitic limestone is grainy (ooids), not banded; banded "onyx" calcite is the same family; alabaster (gypsum) does not fizz	Building stone (Colosseum), floor and wall tiles (honed or polished), decorative facing
Arkose	Clastic	Sand, coarse, angular to subangular; poorly to moderately sorted	Quartz + ≥25% feldspar (pink K-spar); cement calcite, silica, iron oxide	Alluvial fans and rivers close to granite or gneiss source; rapid erosion, little chemical weathering (immature)	Pink to red; visible pink feldspar with flat cleavage faces; gritty	Quartzofeldspathic gneiss or schist (meta-arkose)	Granite has interlocking crystals and breaks through them; arkose has cemented grains with pores and breaks around grains; quartz sandstone lacks feldspar	Building stone, aggregate
Greywacke*	Clastic	Sand, angular; very poorly sorted with >15% clay–silt matrix	Quartz, feldspar, rock (lithic) fragments in clay/chlorite matrix	Deep marine turbidity currents (submarine fans, trenches); graded beds	Dark gray to green-gray; hard, tough; graded bedding; "dirty sandstone"	Phyllite, schist, paragneiss	Arkose is pink and feldspathic with little matrix; quartz sandstone is clean, well sorted, light; basalt shows no sand grains	Crushed stone, road aggregate
Quartz sandstone	Clastic	Sand (1/16–2 mm), well rounded, well sorted	>90% quartz (quartz arenite); cement silica, calcite, iron oxide (red)	Beaches, desert dunes (aeolian), shallow marine; long transport (mature)	Gritty; visible grains; white, tan or red; cross-bedding, ripples; breaks around grains; no fizz unless calcite cement	Quartzite	Quartzite breaks through grains and is sugary-glassy; arkose has feldspar	Glass-making sand, building stone, frac sand; aquifer and oil reservoir

Metamorphic rock table

ReferenceMetamorphic rocks: reference table· 16 rows

• = State/National only. Rows marked "not on list" are comparison rocks that appear in the facies and plate-setting topics. Grade: low ≈200–400 °C, medium ≈400–600 °C, high >600 °C (approximate).

16 rows

Rock	Foliated? texture	Grade	Protolith(s)	Key minerals	Type & setting (regional/contact; plate setting; facies)	Diagnostic features	Look-alikes & how to tell	Uses
Amphibolite*	Weakly foliated or lineated (aligned hornblende needles) to massive; medium to coarse	Medium to high	Basalt, gabbro (ortho-amphibolite); impure calcareous shale (para-amphibolite)	Hornblende + plagioclase ± garnet, epidote, biotite	Regional; metamorphosed oceanic crust in convergent mountain belts; amphibolite facies	Dark green to black; shiny hornblende cleavage faces aligned; heavy; little or no quartz	Gabbro has pyroxene, random orientation; hornblende schist is more strongly foliated	Dimension stone (sold as "black granite"), crushed stone, aggregate
Gneiss	Foliated: gneissic banding (light quartz–feldspar bands alternating with dark biotite/hornblende bands); coarse	High	Granite (orthogneiss); shale, greywacke, arkose (paragneiss); other rocks	Quartz, feldspar, biotite, hornblende ± garnet, sillimanite	Regional; deep crust in continent–continent collision zones; upper amphibolite to granulite facies; gneiss domes (uplifted cores of gneiss) are common in orogenic belts	Distinct light/dark bands, often folded; coarse grains; does not split into sheets	Granite is not banded; schist is platy and mica-rich; migmatite shows melted swirls	Building and dimension stone (often sold as "granite"), aggregate; among the oldest dated rocks (Acasta gneiss ≈4.0 Ga)
Marble	Non-foliated; granoblastic interlocking calcite/dolomite crystals; sugary	Any (low to high)	Limestone, dolostone	Calcite or dolomite ± tremolite, diopside, wollastonite, serpentine (impure marbles)	Regional or contact; convergent belts and contact aureoles	Fizzes in HCl (dolomitic marble weakly); H 3 (knife scratches); sparkly cleavage faces; white when pure, but impurities color it gray or black (organic matter, graphite), pink or red (iron oxide), green (serpentine, chlorite) or brown; often veined	Quartzite is H 7, no fizz; rock gypsum/alabaster is H 2, no fizz; limestone often shows fossils or grains	Sculpture, building stone (Taj Mahal, Lincoln Memorial), countertops, tiles; lime and filler
Phyllite	Foliated; fine grained with silky sheen (phyllitic luster); foliation often wavy/crenulated	Low (to low–medium)	Shale, mudstone (via slate)	Chlorite, sericite (fine muscovite), quartz	Regional; mountain belts; greenschist facies	Satiny sheen; grains invisible; wavy cleavage	Slate is dull and splits flat; schist has visible mica flakes	Decorative and landscaping stone, minor roofing and flooring
Quartzite	Non-foliated; granoblastic fused quartz; sugary to glassy	Any (low to high)	Quartz sandstone; chert	Quartz ± muscovite (green fuchsite), hematite	Regional or contact	H 7 (scratches glass); breaks through grains; no fizz; white, pink, tan, gray	Marble is H 3 and fizzes; sandstone breaks around grains and is gritty	Building stone, countertops, railroad ballast, roofing, high-purity silica source
Garnet schist	Foliated (schistosity) with garnet porphyroblasts; medium to coarse; foliation bends around the rigid garnets because weak micas flow around them during flattening	Medium (to high)	Shale, mudstone (Al-rich)	Muscovite, biotite, quartz, almandine garnet ± staurolite, kyanite	Regional; collisional mountain belts; upper greenschist to amphibolite facies (garnet zone and above)	Red-brown dodecahedral garnets in glittering mica; splits along foliation	Mica schist lacks garnet; gneiss is banded; garnet amphibolite is dark with hornblende	Garnet for abrasives (sandpaper, sandblasting, waterjet cutting); decorative stone
Mica schist	Foliated (schistosity); medium to coarse visible mica flakes	Medium	Shale, mudstone	Muscovite and/or biotite, quartz ± chlorite	Regional; mountain belts; greenschist to amphibolite facies	Glittery, platy, visible flakes; splits readily; often crenulated	Phyllite is finer with sheen only; gneiss is banded, less mica	Landscaping and flagstone; bedrock (Manhattan schist)
Talc schist (soapstone)*	Talc schist foliated; soapstone massive (non-foliated); fine to medium	Low to medium	Ultramafic rock (peridotite, dunite, serpentinite) altered by fluids; some from dolostone	Talc ± chlorite, magnesite, serpentine, tremolite	Regional and hydrothermal (metasomatic); ophiolites at convergent margins; greenschist facies	Soapy/greasy feel; H ≈1 (fingernail scratches); gray to green; heat resistant	Serpentinite is harder (H ≈3–5) and mottled green; rock gypsum is H 2 and white	Carving, countertops and sinks (acid and heat resistant), woodstoves, lab benches; talc powder
Serpentinite*	Non-foliated (massive) to sheared/foliated; slickensided surfaces	Low to medium (water added)	Peridotite, dunite (ultramafic mantle rock)	Serpentine minerals (antigorite, lizardite, chrysotile) ± magnetite, brucite, talc	Hydration (serpentinization) of mantle rock at mid-ocean ridges and hydrothermal vents, in subduction zones; exposed in ophiolites; greenschist facies	Mottled green to black ("snakeskin"); waxy to greasy luster; slippery feel; H ≈3–5; may be magnetic	Soapstone is softer (H 1); jade (nephrite) is H 6–6.5; green marble fizzes	Decorative "green marble" (verde antique); historic chrysotile asbestos; California state rock; Mg source; CO₂ capture research
Slate	Foliated (slaty cleavage); very fine, grains invisible	Low	Shale, mudstone; sometimes volcanic ash	Clay minerals recrystallized to chlorite, sericite/muscovite, quartz	Regional; outer parts of mountain belts; lowest greenschist (or sub-greenschist) facies	Splits into flat thin sheets; dull luster; harder than shale; rings when tapped; cleavage can cut bedding	Shale is softer, earthy, splits along bedding; phyllite has a sheen	Roofing tiles, flooring, chalkboards, billiard tables, gravestones, flagstones
Hornfels (not on list)	Non-foliated; fine, granoblastic; very tough	Low to high temperature at low pressure	Shale, mudstone, basalt, almost any rock	Biotite, cordierite, andalusite, pyroxene, garnet	Contact; aureoles around plutons; hornfels facies	Dark, hard, splintery or conchoidal fracture; may be spotted with porphyroblasts	Basalt has igneous minerals/vesicles; slate splits into sheets; chert is H 7 and waxy	Aggregate; "ringing rocks" and lithophones; prehistoric tools
Migmatite (not on list)	Banded and swirled: light melted bands (leucosome) in dark host (melanosome)	Very high (partial melting)	Gneiss, schist, shale-derived rocks, granite	Quartz, feldspar, biotite, hornblende, sillimanite, garnet	Regional; deepest continental crust in collision zones; upper amphibolite to granulite facies	Half igneous, half metamorphic look; folded, contorted light veins	Gneiss has regular bands without melt veins	Dimension and decorative stone
Greenschist / greenstone (not on list)	Greenschist foliated; greenstone non-foliated	Low	Basalt, gabbro, andesite, mafic tuff	Chlorite, epidote, actinolite, albite	Seafloor (hydrothermal) metamorphism at mid-ocean ridges; low-grade regional; greenschist facies	Green color from chlorite, epidote, actinolite	Serpentinite is waxier and mottled; green slate is very fine and splits flat	Aggregate, decorative stone; Archean greenstone belts host gold ores
Blueschist (not on list)	Foliated; fine to medium	High pressure, low temperature	Basalt (oceanic crust), greywacke	Glaucophane (blue amphibole), lawsonite, jadeite, epidote, garnet	Subduction zones (cold slab carried down fast); blueschist facies (e.g. Franciscan Complex, California)	Blue to lavender-gray color	Ordinary dark schist lacks blue amphibole	Rare decorative use
Eclogite (not on list)	Usually non-foliated (massive); medium to coarse	High to very high pressure, medium to high temperature	Basalt, gabbro	Omphacite (green Na-pyroxene) + red pyrope–almandine garnet; no plagioclase	Deep subduction zones and roots of collision belts; xenoliths in kimberlite; eclogite facies	Red garnets in green matrix; very dense (SG ≈3.4–3.5)	Garnet amphibolite has black hornblende and plagioclase	Decorative stone; some eclogite xenoliths carry diamonds
Skarn (not on list)	Non-foliated; coarse, irregular	Contact (high temperature)	Limestone, dolostone (plus fluids from an intrusion)	Garnet (grossular–andradite), diopside, wollastonite, epidote, magnetite ± ore sulfides	Contact metasomatism at the intrusion–carbonate contact; hornfels-facies temperatures	Coarse green and brown calc-silicates, often with metallic ore minerals	Marble lacks abundant garnet/pyroxene; hornfels is fine and uniform	Ore of tungsten (scheelite), copper, iron, zinc–lead, molybdenum, gold

Rock look-alike pairs

ReferenceRock look-alike pairs· 30 rows

The single observation that separates each confusable pair or trio.

30 rows

Pair	How to tell them apart (decisive observation)
Marble vs quartzite	Drop of dilute HCl: marble fizzes (calcite) and a steel knife scratches it (H 3); quartzite does not fizz and scratches glass (H 7).
Basalt vs gabbro vs diorite	Grain size first: basalt has no visible crystals; gabbro and diorite are coarse. Then color index: gabbro is mostly dark minerals with gray plagioclase; diorite is about half white plagioclase, half black hornblende.
Rhyolite vs andesite	Color and phenocrysts: rhyolite is pink, tan or pale gray and may carry quartz phenocrysts; andesite is medium-dark gray with white plagioclase and black hornblende phenocrysts, no quartz.
Pumice vs scoria	Pumice is pale, has fine glassy froth and usually floats; scoria is dark red or black with larger round vesicles and sinks.
Obsidian vs dark chert/flint	Obsidian has bright glassy luster and is translucent on thin edges (H ≈5.5); chert/flint is waxy-dull, opaque and scratches glass easily (H 7).
Breccia vs conglomerate vs tuff breccia	Clast shape and origin: breccia has angular clasts of any rock; conglomerate has rounded clasts; tuff breccia has angular volcanic blocks set in pale ash with glass shards or pumice.
Arkose vs granite	Granite crystals interlock and the rock breaks through them; arkose grains are angular to subangular, held by cement with pore spaces, and it breaks around grains.
Greywacke vs arkose vs quartz sandstone	Greywacke is dark with >15% muddy matrix and rock fragments; arkose is pink with ≥25% feldspar; quartz sandstone is light, well sorted, >90% rounded quartz.
Shale vs slate vs phyllite	Shale is soft, earthy and splits along bedding; slate is harder, dull, rings when tapped and splits into flat sheets; phyllite has a silky sheen and wavy foliation.
Chalk vs diatomite	Acid test: chalk (calcite) fizzes; diatomite (opal silica) does not and is even lighter.
Travertine vs oolitic limestone	Travertine is banded and porous with vugs (spring or cave deposit); oolitic limestone is made of tiny round sand-size ooids like fish roe.
Coquina vs fossil limestone	Coquina is a loosely cemented, very porous mass of shell fragments; fossil limestone has distinct fossils in a dense, well-cemented fine matrix.
Dolostone vs limestone	Limestone fizzes vigorously on a fresh surface; dolostone fizzes only weakly when scratched into powder.
Rock salt vs rock gypsum	Rock salt has cubic cleavage, H 2.5 and dissolves in water; rock gypsum is H 2 (fingernail scratches) and does not noticeably dissolve.
Anthracite vs bituminous vs lignite	Anthracite is shiny, conchoidal and does not soil fingers; bituminous is banded dull/bright, blocky and leaves black on fingers; lignite is brown, woody and crumbly.
Schist vs gneiss	Schist is platy with abundant visible mica and splits along foliation; gneiss has segregated light and dark bands and does not split into sheets.
Serpentinite vs soapstone	Hardness: soapstone (talc) is H ≈1 and a fingernail scratches it; serpentinite is H ≈3–5 and mottled "snakeskin" green.
Amphibolite vs gabbro	Amphibolite is dominated by shiny, aligned hornblende needles with two cleavages at ≈56°/124°; gabbro has unaligned pyroxene (cleavages near 90°) and plagioclase.
Banded iron formation vs jasper/chert	BIF is heavy with metallic hematite or magnetite bands alternating with red jasper or chert (may attract a magnet); jasper and chert are uniform, light and non-metallic.
Bauxite vs pisolitic limestone	Acid test: pisolitic limestone fizzes; bauxite does not, is earthy and has a clay smell.
Granite vs syenite	Quartz content: granite has ≥20% glassy gray quartz; syenite has under ≈5%.
Granite vs gneiss	Granite has random, unoriented grains; gneiss has its minerals segregated into parallel bands.
Quartz sandstone vs quartzite	Sandstone breaks around grains and feels gritty; quartzite breaks through grains with a glassy, sugary surface.
Tuff vs tuff breccia	Fragment size: tuff is mostly ash under 2 mm; tuff breccia has 25–75% angular volcanic blocks over 64 mm in an ash matrix.
Tuff vs sandstone	Tuff contains glass shards, pumice and angular crystals and is light for its size; sandstone has rounded mineral grains and is heavier.
Shale vs siltstone	Shale is smooth and fissile; siltstone feels faintly gritty and breaks in blocks.
Slate vs hornfels	Slate splits into flat sheets along slaty cleavage; hornfels has no cleavage and breaks into sharp splintery pieces.
Peridotite vs serpentinite	Peridotite is granular, glassy olive-green olivine and dense; serpentinite is waxy, slippery, mottled and softer.
Marble vs rock gypsum (alabaster)	Marble fizzes and is H 3; alabaster does not fizz and is H 2 (fingernail scratches).
Granite vs pegmatite	Same minerals; pegmatite crystals are ≥1 cm and often much larger, sometimes with tourmaline, beryl or book mica.

Rock heft benchmarks

ReferenceRock heft benchmarks· 7 rows

Approximate bulk specific gravity (density in g/cm³) of hand samples; pores lower the value. Heft a known granite first, then compare.

7 rows

Heft	Rocks (bulk SG)
Floats or nearly so (<1)	Pumice (usually floats); dry diatomite (very porous, well under 1)
Very light (≈1.1–1.8)	Coal: lignite ≈1.1–1.3, bituminous ≈1.2–1.5, anthracite ≈1.3–1.8
Light for a rock (≈2.0–2.6)	Porous sedimentary rocks: sandstone ≈2.0–2.6, chalk (often only ≈1.5–2.0 when porous); scoria and tuff are lighter still when very vesicular or porous
"Normal" (≈2.6–2.7)	Felsic igneous (granite, rhyolite), quartzite, marble, dense limestone
Noticeably heavy (≈2.9–3.0)	Mafic igneous (basalt, gabbro), amphibolite; intermediate rocks (diorite, andesite) sit between at ≈2.8
Very heavy (≈3.2–3.3)	Ultramafic: peridotite
Heaviest common rocks (≈3.3–3.8)	Eclogite ≈3.4–3.5 (not on the ID list); banded iron formation ≈3.3–3.8, rising with iron-oxide content (the heaviest list rock)

Identification gallery

Use this gallery to train your eye for the identification station. Cover the captions, name each specimen from the photo alone, then read the caption to check yourself and to learn which one or two tests would settle the call. Names marked * are State and National specimens only. The photos show typical examples, but real specimens vary in color, habit and weathering, so always confirm an identification with hardness, streak, cleavage, acid and the other tests in the reference tables.

Photos: native elements

Photos: sulfides

Photos: halides

Photos: oxides and hydroxides

Photos: carbonates and borates

Photos: sulfates

Photos: phosphates

Photos: silicates, amphiboles to kyanite

Photos: silicates, micas to zircon

Photos: quartz varieties and opal

Photos: igneous rocks

Photos: sedimentary rocks

Photos: metamorphic rocks

Calculations index

Calculations are a small share of Rocks and Minerals questions (about 2% of the question bank), but they are easy points if you have the formula and one worked example in your binder. Graph reading is more common: Bowen's series, the Hjulström diagram, phase diagrams, the facies diagram and the igneous classification chart all appear as "read this chart" questions. Every type is worked in full in its own chapter; this table is the index.

Common pitfalls

Test yourself

Real questions from past tournaments, grouped by chapter. Click an answer to check it; each shows where it came from, and most have a one-line explanation.

Test yourselfTest yourself: Mineral properties and identification

0 / 0 correct

1. 1

   Which mineral reacts with acid?

   Question 1 options

   AQuartzBHaliteCCalciteDFeldspar

   From: Crown Point Invitational - IN, 2025-26, Division B

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Test yourselfTest yourself: Chemistry, groups, silicates and crystal systems

0 / 0 correct

1. 1

   Which mineral is a polymorph of graphite?

   Question 1 options

   AHaliteBOlivineCMicroclineDDiamond

   From: Raisbeck Aviation Invitational - WA, 2025-26, Division C

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Test yourselfTest yourself: Formation, uses and gems

0 / 0 correct

1. 1

   Which mineral is commonly used as a gemstone and is composed of the element aluminum oxide?

   Question 1 options

   ARubyBAmethystCEmeraldDTopaz

   From: Pembroke Hill Invitational, 2025-26, Division B

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Test yourselfTest yourself: Igneous rocks, Bowen's series and melt evolution

0 / 0 correct

1. 1

   What does it mean for a rock to be plutonic?

   Question 1 options

   AA rock that comes from PlutoBA rock that cools on the inside of the earthCA rock that cools exposed to the atmosphereDAny igneous rockEA rock from rapid cooling after being thrown into water or similar cool liquid

   From: CSE (Captains Summer Exchange), 2025-26, Division C

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Test yourselfTest yourself: Sedimentary rocks and weathering

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1. 1

   What type of rock is most likely to contain fossils?

   Question 1 options

   AIgneousBSedimentaryCMetamorphicDAll are equally likely

   From: CrabSO Invitational - MD, 2025-26, Division B

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Test yourselfTest yourself: Metamorphic rocks

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   Igneous and metamorphic rock can be buried and undergo tremendous heating and stress. What is the process of transformation of one rock type into another?

   Question 1 options

   ACompactionBLithificationCMetamorphismDweathering

   From: High Desert Satellite Invitational, 2022-23, Division C

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Test yourselfTest yourself: The rock cycle and dating

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1. 1

   Rocks are always made of only one mineral.

   Question 1 options

   ATrueBFalse

   From: Cobra Invitational - MI, 2025-26, Division B

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Condensed facts

These pages condense the guide into the sheets most teams put at the front of their binder: identification keys first (metallic minerals by streak, non-metallic minerals by hardness band, habits; the rock key opens the rock pages), then the concept tables, look-alikes, formulas and gems. * = State/National only; ¹ = supervisor supplies diagnostic information. H = Mohs hardness, SG = specific gravity, LW/SW = long-/short-wave UV. For any specimen, run luster → streak → hardness → cleavage → acid → heft.

Metallic minerals by streak

Every metallic or submetallic list mineral, dark streak to light. No list silicate is metallic.

• Brassy trio: gold (yellow streak, dents) vs pyrite (H 6–6.5) vs chalcopyrite (H 3.5–4, iridescent tarnish).
• All can be black: red-brown streak = hematite; black + magnetic = magnetite; yellow-brown = goethite/limonite.
• Light metallic: graphite 2.2 vs galena 7.5. Heft separates them at once.

Non-metallic minerals by hardness band

Streak is white unless stated. Fingernail 2.5, glass/knife 5.5. SG = specific gravity (heft): most non-metallics are 2.6–3.0, so anything near 4 or above feels heavy.

H under 2.5: fingernail scratches it

H 2.5 to 5.5: fingernail fails, does not scratch glass

H over 5.5: scratches glass

Mohs scale and test objects

• Mnemonic: Tall Giraffes Can Fight And Often Quarrel Till Completely Defeated. Mohs is ordinal; the 9 → 10 step is the largest. Never add or take ratios of Mohs numbers.
• Minerals harder than the plate (~7) leave no streak: quartz varieties, topaz, corundum, garnet, beryl, zircon, diamond.
• Direction-dependent hardness: kyanite (4.5–5 along the blade, 6.5–7 across).

Luster, cleavage and fracture terms

• Fracture: conchoidal (quartz, obsidian, chert, opal); hackly (Cu, Ag, Au); splintery/fibrous (satin spar, kyanite, asbestiform amphiboles); earthy (kaolinite, limonite, bauxite, chalk).
• Parting looks like cleavage but only some specimens show it: corundum (basal, rhombohedral), magnetite (octahedral).
• Tenacity: brittle (most); malleable/ductile/sectile (Au, Ag, Cu); flexible, stays bent (chlorite, talc, selenite); elastic (muscovite, biotite, lepidolite).

Habit terms and twin types

The rules' closed list of 21 habits, one line each (descriptive extras such as fibrous or reniform are in the Mineral terms glossary).

The eight other properties

The rules close this list at eight. Tournaments supply acid and taste results in words.

• Related terms: phosphorescence (glow continues; some calcite, willemite, fluorite), thermoluminescence (chlorophane fluorite), triboluminescence (quartz pebbles rubbed, some sphalerite), tenebrescence (hackmanite sodalite).

Families and anion groups

• Crust by mass: O 46.6, Si 27.7, Al 8.1, Fe 5.0, Ca 3.6, Na 2.8, K 2.6, Mg 2.1%. Silicates are over 90% of the crust; feldspars about half.
• Sulfide (S²⁻, metallic) ≠ sulfate (SO₄²⁻, non-metallic). Fluorite is a halide; fluorapatite is a phosphate.
• Ore grades: hematite 69.9% Fe; magnetite 72.4% Fe; galena 86.6% Pb; chalcopyrite 34.6% Cu. Gypsum is about 21% water by mass.

Silicate structure classes

• O per Si = 4 − s/2; charge per Si = −(4 − s), where s = corners shared.
• Rings and single chains tie at 1 : 3. More polymerized = later in Bowen's series, more weathering-resistant.
• Sheet layers: 1 : 1 (T–O) kaolinite, serpentine; 2 : 1 (T–O–T) talc, micas; chlorite 2 : 1 + hydroxide sheet. Asbestos: chrysotile plus five amphiboles (tremolite, actinolite on the list).

Crystal systems

• Cubic = isotropic (no double refraction or pleochroism; black in XPL). Tetragonal, hexagonal, trigonal = uniaxial; the other three = biaxial.
• Monoclinic is the most common system; cubic is the most symmetrical, triclinic the least. Quartz looks hexagonal but is trigonal.

Polymorphs and solid solutions

• Plagioclase by % An: albite 0–10, oligoclase 10–30, andesine 30–50, labradorite 50–70, bytownite 70–90, anorthite 90–100.
• Other series: garnets (almandine–pyrope; grossular–andradite), biotite (annite–phlogopite), sphalerite (Zn with up to ~¼ Fe), tourmaline (schorl, dravite, elbaite).
• Polymorph = same formula, different structure. Solid solution or isostructural = same structure, formula varies. Isostructural sets: calcite group (calcite, magnesite, siderite, rhodochrosite); barite–celestite; apatite–pyromorphite–vanadinite; halite–galena; almandine–grossular.
• Al₂SiO₅ triple point ≈500 °C, ≈4 kbar (0.4 GPa).

Rock identification key

Texture first, then composition. Start at 1 and follow the first line that fits; every list rock is an end point. * = State/National only; ¹ = supervisor supplies data.

Igneous classification grid

• Pairs: rhyolite ↔ granite, andesite ↔ diorite, basalt ↔ gabbro (diabase in dikes and sills), trachyte ↔ syenite, komatiite ↔ peridotite.
• Granite ≥20% quartz; syenite and diorite under ≈5%. Only granite, rhyolite and pegmatite on the list carry >5% visible quartz.
• Felsic → ultramafic: SiO₂, Na, K, Al fall; Fe, Mg, Ca rise; darker, denser, hotter, less viscous. Densities ≈2.6–2.7 → 2.8 → 3.0 → 3.3.
• Color-index vocabulary (IUGS, a separate scale from the working ranges above): hololeucocratic 0–10, leucocratic 10–35, mesocratic 35–65, melanocratic 65–90, holomelanocratic 90–100.
• QAPF (Div C): Q quartz, A alkali feldspar, P plagioclase, F feldspathoids; for plutonic rocks; Q and F never together.

Bowen's series order

• Down the series: T falls; polymerization, Si, Na, K, Al rise; Fe, Mg, Ca fall; lighter color and density; residual melt more viscous; more weathering-resistant (Goldich).
• Discontinuous = different minerals; continuous = one mineral, changing composition. Quartz is last; biotite is last of the discontinuous branch. Amphibole and biotite need water.
• Minerals that crystallize together occur together. Olivine + quartz means two events (xenoliths, mixing, a later vein, faulting).
• Normal zoning = Ca core, Na rim. Reaction rim = pyroxene around olivine.

Igneous textures and cooling

• Euhedral = early; anhedral = late (quartz in granite). Holocrystalline (plutonic), hypocrystalline (lavas), holohyaline (obsidian).
• Amygdules = filled vesicles. Xenolith = foreign rock fragment. Chilled margins = fast-cooled edges. Columnar joints, pillow lava (underwater), flow banding.
• Magma: basaltic ≈1000–1200 °C, low viscosity, little gas, effusive; andesitic ≈800–1000 °C; rhyolitic ≈650–800 °C, most viscous and gassy, explosive. Volcanoes: shield (basalt), composite/strato (andesite), cinder cone (gassy basalt), lava dome (degassed rhyolite), caldera (collapse).
• Ejecta: ash <2 mm, lapilli 2–64 mm, blocks >64 mm (solid), bombs >64 mm (molten).

Igneous bodies and plate settings

The 2027 rules name only batholith, lava flow and pyroclastic deposits.

• Volume made: decompression (ridges) > flux (subduction) > heat transfer and hot spots.
• Ophiolite, top down: deep-sea sediment, pillow basalt, sheeted dikes, gabbro, peridotite (often serpentinized).
• Div C melt evolution: partial melting, fractional crystallization, assimilation and mixing all push a melt more felsic. Incompatible elements (K, Rb, Cs, Li, Be, B, U, Th, Zr, Nb, Ta, REE) collect in pegmatite*.

Sediment sizes, textures and cements

• φ = −log₂(d in mm); bigger grain = more negative φ. Mud = silt + clay.
• Textures (closed list of three): clastic, chemical, biochemical. Bioclastic (coquina) = biochemical origin, clastic texture.
• Grain size = energy; rounding = distance; sorting = steadiness of energy. Wind: best sorted, frosted. Ice and debris flows: unsorted, angular.
• Hjulström: sand (≈0.1–0.5 mm) is easiest to erode (≈20–30 cm/s); clay is cohesive and hard to erode, stays suspended, settles only in still water.
• Lithification = compaction (dominant for mud) + cementation (dominant for sand and gravel) + recrystallization.

• Maturity: greywacke* (>15% matrix) → arkose (≥25% feldspar) → quartz sandstone (>90% quartz).
• Evaporite order: calcite (~50% water left) → gypsum/anhydrite (~20%) → halite (~10%) → K–Mg salts (<5%).
• Coal rank and carbon (USGS, as-mined basis): lignite 25–35 → sub-bituminous 35–45 → bituminous 45–86 → anthracite 86–97. Peat (not a rock) is ≈50–60% carbon only on a dry basis, so do not compare it with these figures. Practically no coal is older than the Devonian; most is Carboniferous.

Sedimentary rocks and their settings

• Way-up indicators: graded beds, cross-beds (truncated top), mud cracks, symmetric ripples, raindrop pits, sole marks.
• Sandstone → shale → limestone upward = transgression; shale → sandstone → coal upward = regression. Division C calls the deep sea abyssal.

Weathering and mineral stability

• First to crystallize = first to weather; that is why sand is mostly quartz.
• Mechanical: frost wedging (~9% expansion), exfoliation (unloading), salt growth, roots, abrasion, thermal. Chemical: hydrolysis, oxidation, dissolution, carbonation, hydration.
• 2KAlSi₃O₈ + 2H₂CO₃ + 9H₂O → Al₂Si₂O₅(OH)₄ + 4H₄SiO₄ + 2K⁺ + 2HCO₃⁻ (K-feldspar → kaolinite).
• Granite's fate: quartz → sand; feldspars → clay; biotite, hornblende → clay + iron oxide; muscovite weathers slowly.

Metamorphic protoliths and grade

• Foliation sequence (shale): slate (slaty cleavage, dull, rings) → phyllite (silky sheen, wavy) → schist (visible mica, porphyroblasts) → gneiss (light/dark bands, does not split) → migmatite (partial melt).
• Non-foliated: marble, quartzite, hornfels (contact), and any rock of equidimensional grains or formed without directed stress.
• Agents: heat (most important), confining and directed pressure, fluids (metasomatism). ≈1 kbar per 3.5 km; geotherm ≈25–30 °C/km.

• Closed list of seven. Isograd = line where an index mineral first appears. Andalusite replaces kyanite at low P (Buchan, contact). At high grade muscovite + quartz → sillimanite + K-feldspar + water.

Facies and settings (Division C)

• Geotherms: steep → hornfels (contact); moderate (≈25–30 °C/km) → greenschist → amphibolite → granulite (Barrovian); low (≈10 °C/km) → blueschist → eclogite.
• Regional = large area, directed stress, foliated, convergent. Contact = aureole, high T, low P, non-foliated hornfels. Dynamic = fault breccia, mylonite. Shock = coesite, stishovite, shatter cones.

Thin-section summary (Division C, State and National)

• PPL (one polarizer): color, pleochroism, relief, cleavage, crystal shape. XPL (crossed polarizers): interference color (birefringence), extinction, twinning.
• Section 30 µm thick; retardation Δ = d × δ. Quartz ~270 nm (first-order white), augite ~750 nm (second order), calcite ~5,160 nm (high-order white).
• Isotropic (cubic minerals, glass) = black at every angle. Anisotropic grains go black every 90° of rotation; low birefringence looks gray, not black.
• Textures: holocrystalline (all crystals) vs hypocrystalline (crystals + glass) vs glassy (black in XPL); vesicular = round holes filled with (often blue) epoxy; porphyritic = phenocrysts in fine groundmass.
• Sediments: quartz arenite = >90% rounded, sorted quartz, little matrix; arkose = ≥25% twinned or tartan feldspar, angular; ooids = concentric coated grains; skeletal fragments = shells, crinoid plates.

Rock look-alike tests

Mineral look-alike tests

Formulas and calculations

• Zircon: takes U⁴⁺, rejects Pb²⁺, two U clocks, closure >900 °C, survives weathering. Oldest material: Jack Hills zircon ≈4.4 Ga; oldest rock: Acasta Gneiss ≈4.0 Ga; Earth ≈4.54–4.57 Ga (meteorites).
• Closure T (Div C): zircon U–Pb >900 °C; hornblende Ar ≈500–550; muscovite ≈350–400; biotite ≈300–350; K-feldspar ≈150–300 °C.
• Relative dating: superposition, original horizontality, lateral continuity, cross-cutting (cutter younger), inclusions (older), contact baking, faunal succession. Unconformities: angular, nonconformity (on igneous/metamorphic), disconformity, paraconformity.
• Decay: α (Z −2, A −4); β⁻ (Z +1); electron capture (Z −1). K-40 → Ar-40 is electron capture (~11%); β⁻ gives Ca-40.

One-line traps

• P : D = 1 : 7 is 3 half-lives, not 7. "75% decayed" means 25% remains.
• Age = n × half-life, not n. Watch million vs billion.
• SG has no units; density does. SG uses weight lost in water.
• Pores lower measured SG. Weighing in oil does not give SG.
• Convert masses to atoms before D/P when atomic masses differ.
• C-14 never dates rocks; Rb–Sr never dates young samples.
• Lever rule: the phase closer to the bulk composition is more abundant. Tie lines are horizontal.
• Mohs numbers are never added, averaged or divided.
• 1 carat = 0.2 g; karat measures gold purity.

Gem varieties and color causes

• Idiochromatic (element in formula, color survives powdering): malachite, azurite, turquoise (Cu²⁺); rhodochrosite, rhodonite (Mn²⁺); almandine, peridot (Fe²⁺); sulfur. Allochromatic: quartz, corundum, beryl, spodumene, fluorite, topaz. Pseudochromatic: opal, labradorite, bornite tarnish.
• Color clues: red/green gem → Cr; pink → Mn; blue-green → Cu (Fe in beryl); purple or smoky quartz → irradiation.
• Precious: diamond, ruby, sapphire, emerald. Pearl and amber are organic, not minerals. "Herkimer diamonds" (quartz) and "Matara diamonds" (zircon) are misleading trade names; cubic zirconia is not zircon.
• Phenomena: play of color (opal), chatoyancy (tiger's eye, satin spar), asterism (star ruby, sapphire, rose quartz; rutile needles), adularescence (moonstone), aventurescence (aventurine), fiber-optic (ulexite).

References and further reading

• Steven Earle, Physical Geology (2nd ed., open textbook): minerals, igneous, sedimentary and metamorphic rocks, weathering and geological time: https://opentextbc.ca/physicalgeology2ed/
• Handbook of Mineralogy (Mineralogical Society of America), one PDF per mineral with crystal data and physical properties: https://www.handbookofmineralogy.org
• Mindat, mineral database with photos, chemistry and physical properties: https://www.mindat.org
• Webmineral, mineral data tables: http://webmineral.com
• Smith College petrography pages (minerals in thin section): https://www.science.smith.edu/geosciences/petrology/petrography/
• USGS, mineral commodity summaries, volcano hazards and geochronology: https://www.usgs.gov
• Science Olympiad Rocks and Minerals event page (rules, National List and official resources): https://www.soinc.org
• Practice more: the Scio.ly test archive has hundreds of public Rocks and Minerals tests, and the Dynamic Planet guide covers the Earth-science background behind many of the same processes.