Master diagnostic rock and mineral identification across all 27 geological subgroups—7 Igneous cooling/silica families, 6 Sedimentary clastic & chemical regimes, 6 Metamorphic foliation & subduction facies, and the 8 Dana/Strunz chemical mineral classes—paired with an interactive 124-specimen Geology Field Guide.
Igneous rocks solidify from molten magma or lava. To identify any igneous rock in the field, evaluate two axes simultaneously: 1. Cooling Rate / Texture (coarse-grained phaneritic slow underground cooling vs. fine-grained aphanitic, glassy, or pyroclastic rapid surface eruption) and 2. Silica (SiO₂) Composition & Color Index governed by Bowen's Reaction Series.
| Texture / Cooling Regime | Felsic (>69% SiO₂ • Light) | Intermediate (52–69% SiO₂) | Mafic (45–52% SiO₂ • Dark) | Ultramafic (<45% SiO₂ • Mantle) |
|---|---|---|---|---|
| Intrusive Phaneritic (Slow plutonic cooling; visible interlocking crystals) | Granite, Pegmatite, Aplite, Granodiorite | Diorite, Monzonite, Syenite, Tonalite | Gabbro, Norite, Anorthosite, Troctolite, Diabase | Peridotite (Dunite, Lherzolite), Pyroxenite, Kimberlite |
| Extrusive Aphanitic / Porphyritic (Rapid volcanic lava cooling) | Rhyolite, Dacite | Andesite, Trachyte, Latite | Basalt (Pillow, Flood, Pāhoehoe), Tachylite | Komatiite (Archean spinifex lava) |
| Glassy / Vesicular / Pyroclastic (Quenched or explosive eruption) | Obsidian, Pumice, Perlite | Welded Tuff (Ignimbrite), Accretionary Lapilli | Scoria, Pele's Hair, Hyaloclastite, Volcanic Bomb | Kimberlite Diatreme Breccia |
Light-colored (leucocratic), low-density (~2.65 g/cm³) intrusive rocks with visible interlocking crystals of translucent gray quartz (>20%), pink/white alkali & plagioclase feldspar, and minor mica or hornblende.
High-viscosity, silica-rich lavas that either cool into pale microcrystalline rhyolite/dacite with quartz phenocrysts, quench instantaneously into razor-sharp conchoidal obsidian glass, or froth into buoyant pumice (SG < 1.0).
Generated above subduction zones. Intrusive diorite displays a classic 50/50 "salt-and-pepper" mosaic of white plagioclase and black hornblende/biotite with little quartz; extrusive andesite shows boxy plagioclase phenocrysts in a gray groundmass.
Dark, dense (~2.9–3.1 g/cm³) plutonic rocks crystallized in mid-ocean ridge magma chambers and layered intrusions. Dominated by calcium-rich labradorite plagioclase and pyroxene (augite/orthopyroxene) with olivine and zero quartz.
Low-viscosity, iron- and magnesium-rich lavas erupting at >1,100 °C. Includes dark microcrystalline basalt, cindery scoria (sinks in water, unlike pumice), subaqueous pillow basalt, and ropy pāhoehoe.
Extremely dense (~3.3 g/cm³), olive-green to black mantle rocks composed of >90% ferromagnesian minerals (forsterite olivine and enstatite/diopside pyroxene), plus volatile-rich kimberlite pipes that transport diamonds from >150 km depth.
Formed by explosive volcanic fragmentation rather than effusive lava flow. Identified by flattened glassy pumice lenses (fiamme) in welded ignimbrite, angular lava blocks in volcanic breccia, or aerodynamic volcanic bombs.
Sedimentary rocks cover ~75% of Earth's continental land surface and preserve the fossil and paleoclimate record. First determine whether the rock is Detrital / Siliciclastic (cemented mineral and rock fragments classified by the Wentworth Grain-Size Scale) or Chemical / Biogenic (precipitated from solution or built by organisms).
Contains gravel-sized clasts (>2 mm). Inspect clast rounding: smooth water-worn pebbles indicate conglomerate (rivers/beaches), razor-sharp angular fragments indicate breccia (faults/debris flows), and unsorted glacial rock flour with striated cobbles indicates tillite.
Gritty to the touch with visible sand grains. Pure rounded quartz (>95%) is quartz arenite; >25% pink potassium feldspar is arkose; poorly sorted dark sand in a muddy matrix from submarine turbidity currents is greywacke; emerald-green authigenic mica pellets mark marine greensand.
Grains too small to see with the naked eye. Rocks that split along thin paper-like laminations (fissility) are shales; blocky non-fissile clay rocks are mudstones; slightly gritty silt-sized rocks (4–62.5 μm) are siltstones.
Calcite limestones (CaCO₃, Mohs 3) fizz vigorously in cold dilute HCl, while dolostone (CaMg(CO₃)₂) fizzes only when powdered. Includes fossiliferous limestone, spherical-grain oolitic limestone, shell-hash coquina, microscopic coccolith chalk, hot-spring travertine, and microbial stromatolites.
Precipitated sequentially as seawater or playa brine evaporates: calcium sulfate precipitates first as soft rock gypsum (Mohs 2, fingernail-scratchable) and anhydrite, followed by cubic rock salt (halite) at ~10% original volume and bitter potassium-rich potash (sylvite) at <2%.
Includes microcrystalline silica chert/flint (Mohs 7, conchoidal fracture, no acid fizz), porous frustule diatomite, Precambrian banded iron formation (BIF), concentric oolitic ironstone, marine phosphorite, and plant-derived bituminous coal.
Metamorphic rocks recrystallize in the solid state under elevated pressure (P), temperature (T), and chemically active fluids without melting. First check for Foliation (planar alignment of platy micas or compositional bands caused by directed tectonic compression) versus Non-Foliated / Granoblastic equidimensional crystal mosaics.
The first steps in the prograde regional metamorphism of shale/mudstone. Slate splits into flat, matte roof tiles with microscopic chlorite/illite; slightly hotter phyllite develops a wavy, silky silver-green sheen from aligned fine sericite mica.
Defined by macroscopically visible, glittering sheets of muscovite, biotite, chlorite, or talc (schistosity) that wrap around large porphyroblasts of red almandine garnet, twinned staurolite crosses, or blue-bladed kyanite.
At high temperatures, minerals segregate into alternating light (quartz + feldspar) and dark (biotite + hornblende/pyroxene) gneissic bands. Eye-shaped feldspar porphyroclasts mark augen gneiss; partial melting (>650 °C) produces swirled migmatite.
Lacks planar foliation either because metamorphism occurred without directed stress in a thermal contact aureole (hornfels, skarn) or because the protolith was monomineralic: limestone recrystallizes into marble (Mohs 3, fizzes in acid) and sandstone fuses into quartzite (Mohs 7, scratches steel).
Diagnostic of cold oceanic slabs plunged rapidly into subduction trenches. Basalt transforms first into indigo-blue glaucophane blueschist, and deeper (>45 km) into dense red-and-green eclogite (pyrope-almandine garnet + grass-green omphacite clinopyroxene, zero plagioclase).
Formed by hot fluid alteration or intense fault shearing: mantle peridotite hydrates into waxy green serpentinite; ductile deep fault zones stretch crystals into ribboned mylonite; brittle upper-crustal faults crush rock into cataclasite or friction-melted pseudotachylite.
A mineral is a naturally occurring, inorganic crystalline solid with a definite chemical formula and ordered atomic lattice. Mineralogists classify all >5,900 IMA-recognized minerals into 8 major chemical groups based on their dominant negatively charged anion or polyatomic anion group, and identify them in hand sample using Mohs hardness, cleavage vs. fracture, streak, luster, and specific gravity.
Built from silicon–oxygen tetrahedra ((SiO₄)⁴⁻) linked into isolated islands (olivine, garnet, zircon, topaz, kyanite), rings (beryl, tourmaline), chains (augite, hornblende — 90° vs. 56°/124° cleavage), sheets (muscovite, biotite, talc — 1 basal cleavage), or 3D frameworks (quartz, orthoclase, albite — Mohs 6–7).
Built around the planar carbonate ion ((CO₃)²⁻). Diagnostic hallmarks include moderate hardness (Mohs 3–4), three-directional rhombohedral cleavage (75°/105°), and effervescence (CO₂ fizzing) in dilute HCl, plus vivid copper secondary carbonates (malachite green, azurite blue) and pink rhodochrosite.
Metals bonded directly to oxygen (O²⁻) or hydroxyl (OH⁻). Includes strongly magnetic magnetite (Fe₃O₄), red-streak hematite (Fe₂O₃), Mohs 9 hexagonal corundum (Al₂O₃, ruby/sapphire), octahedral spinel, tin ore cassiterite, titanium rutile, and aluminum ore bauxite.
Metals bonded to reduced sulfur (S²⁻) in hydrothermal veins. Most display brassy or lead-gray metallic luster and dark streaks: cubic Mohs 6.5 pyrite, heavy silver-cubic galena (PbS, SG 7.5), resinous sphalerite (ZnS), brass-yellow chalcopyrite (CuFeS₂), crimson cinnabar (HgS), and flexible molybdenite.
Built around oxidized hexavalent sulfur ((SO₄)²⁻). Typically translucent with vitreous to pearly luster: Mohs 2 gypsum (selenite/desert rose), anhydrous anhydrite, unusually heavy baryte (BaSO₄, SG 4.5), sky-blue celestine (SrSO₄), and bright blue triclinic chalcanthite.
Ionic salts of halogen anions (F⁻, Cl⁻). Characterized by isometric symmetry and diagnostic cleavage: Mohs 4 fluorite (CaF₂) splits along 4 octahedral directions, while water-soluble halite (NaCl) and bitter sylvite (KCl) split into perfect 90° cubes.
Tetrahedral (PO₄)³⁻, (AsO₄)³⁻, or (VO₄)³⁻ groups. Includes Mohs 5 hexagonal apatite (the mineral of vertebrate teeth and bones), sky-blue turquoise, neon-green pyromorphite, blazing red hexagonal vanadinite, and rare-earth monazite.
Elements occurring uncombined in nature. Metals (gold, silver, copper) exhibit hackly fracture, extreme malleability, and high density; semimetals (bismuth) form iridescent hopper crystals; nonmetals include yellow orthorhombic sulfur and the two polymorphs of carbon—Mohs 1.5 sheeted graphite vs. Mohs 10 tetrahedral diamond.
Filter by Broad Group (Igneous, Sedimentary, Metamorphic, Minerals), any of the 27 Subgroups, or LithoClade discovery status. Click any specimen card to open its full petrographic, chemical, and Wikimedia Commons license dossier.