Rock Identification Reference

Rock identification entries

• Agglomerate — Coarse pyroclastic rock composed of rounded volcanic bombs 64 mm in a finer tuff matrix.
• Amphibolite — Dark, medium to coarse-grained metamorphic rock composed mainly of hornblende and plagioclase. May be foliated or massive.
• Andesite — Gray to dark gray fine-grained volcanic rock named after the Andes. The extrusive equivalent of diorite.
• Anorthosite — Intrusive igneous rock composed almost entirely 90% of plagioclase feldspar. Common on the Moon.
• Anthracite — Hard, lustrous, high-rank coal with 90% carbon. The highest grade of coal with a semi-metallic luster.
• Aplite — Fine-grained, light-colored igneous rock of granitic composition forming thin dikes. Sugary texture.
• Arkose — Sandstone containing 25% feldspar, indicating rapid erosion of granitic source rocks and minimal chemical weathering.
• Augen Gneiss — Gneiss containing large eye-shaped augen feldspar porphyroclasts in a foliated matrix of finer minerals.
• Banded Iron Formation — Distinctive Precambrian sedimentary rock with alternating layers of iron-rich hematite/magnetite and silica-rich chert bands.
• Basalt — A fine-grained extrusive igneous rock that forms from the rapid cooling of lava. It is the most common rock type on Earth's surface, forming most ocean…
• Basanite — Olivine-bearing alkaline volcanic rock similar to tephrite but with significant olivine. Rich in feldspathoids.
• Bentonite — Soft, swelling clay rock formed from alteration of volcanic ash. Composed mainly of montmorillonite/smectite clays.
• Blueschist — Blue-colored metamorphic rock formed under high pressure and low temperature in subduction zones. Contains glaucophane.
• Bluestone — Term used for various dense, blue-gray stones used in construction. Stonehenge bluestones are dolerite from Wales.
• Boninite — Unusual high-magnesium, low-titanium volcanic rock found exclusively in forearc settings of subduction zones.
• Breccia (Sedimentary) — Coarse-grained sedimentary rock composed of angular fragments 2 mm cemented together. Angular clasts indicate minimal transport.
• Buchite — Partially fused rock formed by extreme contact metamorphism immediately adjacent to igneous intrusions or by lightning strikes.
• Calc-silicate Rock — Metamorphic rock formed from impure limestone or dolostone containing silicate minerals like diopside and wollastonite.
• Carbonatite — Rare igneous rock containing 50% carbonate minerals. The only silicate-poor igneous rock; source of rare earth elements.
• Cataclasite — Fault rock formed by brittle fragmentation of rock along fault zones at shallow depths. Angular fragments in a fine matrix.
• Chalk — Soft, white, fine-grained limestone composed primarily of microscopic coccolithophore algae skeletal remains.
• Charnockite — Orthopyroxene-bearing metamorphic or igneous rock. Named after Job Charnock, founder of Calcutta.
• Chert (Rock) — Hard, dense chemical sedimentary rock composed of microcrystalline quartz. Flint is the dark variety in chalk.
• Claystone — Very fine-grained clastic rock composed predominantly of clay-sized particles <0.004 mm . Non-fissile.
• Coal — Combustible sedimentary rock formed from accumulated plant material. Ranked by carbon content: peat → lignite → bituminous → anthracite.
• Conglomerate — A coarse-grained sedimentary rock composed of rounded pebbles, cobbles, and boulders cemented together.
• Coquina — Poorly cemented sedimentary rock composed almost entirely of shell fragments and other bioclasts.
• Dacite — Fine-grained volcanic rock intermediate between andesite and rhyolite. Common product of explosive eruptions.
• Diabase — Medium-grained mafic intrusive rock also called dolerite with ophitic texture. Forms sills and dikes.
• Diamictite — Non-genetic term for poorly sorted rock with clasts of various sizes in a fine matrix. May be glacial, volcanic, or mass-flow.
• Diatomite — Very light, porous sedimentary rock composed of microscopic diatom siliceous algae frustules.
• Diorite — Coarse-grained intrusive igneous rock intermediate between granite and gabbro. Contains plagioclase, hornblende, and biotite.
• Dolomitic Limestone — Limestone containing significant 10-50% dolomite, representing partial dolomitization of original limestone.
• Dolostone — Sedimentary rock composed primarily of the mineral dolomite CaMg CO₃ ₂. Often forms by replacement of limestone.
• Dunite — An ultramafic rock composed of over 90% olivine. Named after Dun Mountain in New Zealand.
• Eclogite — Dense, colorful rock composed of red garnet and green omphacite pyroxene. Forms at extreme pressures in subduction zones.
• Epidosite — Rock composed almost entirely of epidote and quartz, formed by intense hydrothermal alteration in oceanic crust.
• Essexite — Alkaline gabbro containing nepheline or analcime in addition to pyroxene and plagioclase.
• Evaporite — General term for sedimentary rocks formed by precipitation from evaporating water bodies. Includes multiple mineral phases.
• Fenite — Alkali-metasomatized rock found around carbonatite and alkaline intrusions. Enriched in sodium and potassium.
• Flint — Dark variety of chert found as nodules in chalk and limestone. Used since the Stone Age for tools and fire-starting.
• Flysch — Thick sequence of alternating sandstone/greywacke and shale deposited in deep marine foreland basins during mountain building.
• Fossiliferous Limestone — Limestone containing abundant visible fossil remains including shells, corals, crinoids, and other organisms.
• Gabbro — Dark, coarse-grained intrusive rock rich in pyroxene and calcium plagioclase. The plutonic equivalent of basalt.
• Garnet Schist — Mica schist containing conspicuous garnet porphyroblasts, indicating medium to high grade metamorphism.
• Gneiss — A high-grade metamorphic rock with distinctive banding of light and dark minerals. Represents rocks that have been subjected to extreme heat and pressure…
• Gondite — Manganese-rich metamorphic rock composed mainly of spessartine garnet and quartz. Important manganese ore.
• Grainstone — Grain-supported limestone lacking mud matrix. Well-sorted and cemented with sparry calcite, indicating high-energy environment.
• Granite — A coarse-grained intrusive igneous rock composed mainly of quartz, feldspar, and mica. Granite is one of the most common rocks in the continental crust…
• Granitic Gneiss — Banded metamorphic rock of granitic composition with alternating light felsic and dark mafic layers.
• Granodiorite — Coarse-grained intrusive rock between granite and diorite. More plagioclase than alkali feldspar. Very common in continental crust.
• Granulite — High-temperature metamorphic rock formed at the base of the continental crust. Lacks hydrous minerals like mica.
• Greenschist — Green-colored metamorphic rock characteristic of the greenschist facies. Color from chlorite, epidote, and actinolite.
• Greenstone — General term for dark green, low-grade metamorphic rocks derived from mafic volcanic rocks. Common in Archean terranes.
• Greisen — Quartz-mica rock formed by hydrothermal alteration of granite by fluorine and boron-rich fluids. Associated with tin-tungsten deposits.
• Greywacke — Poorly sorted, dark sandstone with abundant rock fragments and clay matrix. Characteristic of turbidite sequences.
• Gypsum Rock — Sedimentary rock composed primarily of gypsum CaSO₄·2H₂O . Commonly associated with other evaporite minerals.
• Harzburgite — Ultramafic rock composed of olivine and orthopyroxene. A depleted mantle rock that has lost its basaltic component.
• Hawaiite — Mildly alkaline basaltic rock transitional between alkali basalt and mugearite. Common on oceanic islands.
• Hornblendite — Dark ultramafic rock composed almost entirely of hornblende amphibole. Uncommon but found in some plutonic complexes.
• Hornfels — Hard, fine-grained, non-foliated metamorphic rock formed by contact metamorphism adjacent to igneous intrusions.
• Icelandite — Iron-rich intermediate volcanic rock transitional between tholeiitic basalt and rhyolite. Characteristic of Iceland.
• Ignimbrite — Welded tuff formed from high-temperature pyroclastic flows. Fragments fuse together due to intense heat during emplacement.
• Impactite — Rock formed by the melting and resolidification of target rocks during a meteorite impact. Contains shocked minerals.
• Ironstone — Sedimentary rock containing 15% iron, typically as siderite, hematite, or goethite. Important historical iron ore.
• Itabirite — Metamorphosed banded iron formation with alternating bands of recrystallized hematite/magnetite and quartz.
• Jade (Nephrite rock) — Tough metamorphic rock composed of interlocking actinolite-tremolite fibers. One of two minerals called jade.
• Kimberlite — Rare ultramafic volcanic rock that originates from depths 150 km. The primary source of natural diamonds.
• Komatiite — Rare ultramafic volcanic rock with distinctive spinifex texture. Formed from extremely hot lavas 1600°C mostly in the Archean.
• Lamprophyre — Dark, porphyritic dike rock rich in biotite or hornblende phenocrysts in a fine-grained groundmass.
• Lapis Lazuli (rock) — Deep blue metamorphic rock prized since antiquity. Composed of lazurite, calcite, and pyrite.
• Laterite — Iron and aluminum-rich residual soil/rock formed by intense tropical weathering. Often red-brown and pisolitic.
• Latite — Fine-grained volcanic rock with roughly equal amounts of plagioclase and alkali feldspar, minimal quartz.
• Lherzolite — Fertile mantle peridotite containing olivine, orthopyroxene, and clinopyroxene. Represents undepleted upper mantle.
• Lignite — Brownish-black low-rank coal with visible plant structures. The lowest rank of true coal with 60-70% carbon content.
• Limestone — A sedimentary rock composed primarily of calcium carbonate calcite . It often contains fossils and is an important industrial rock used in construction…
• Loess — Unstratified, buff-colored silt deposited by wind, typically from glacial outwash plains. Forms vertical cliffs when eroded.
• Marble — A metamorphic rock formed from the recrystallization of limestone under heat and pressure. Prized for its beauty and used in sculpture and architecture…
• Marble (Cipollino) — Banded metamorphic rock with alternating layers of calcite marble and green mica/chlorite-rich bands. Decorative stone.
• Marble (Dolomitic) — Metamorphic rock formed from dolostone. Contains dolomite rather than calcite as the primary carbonate mineral.
• Marl — Sedimentary rock composed of a mixture of clay minerals 35-65% and calcium carbonate 35-65% .
• Metabasalt — Low to medium grade metamorphosed basalt retaining some original volcanic texture but with new metamorphic minerals.
• Metagraywacke — Weakly metamorphosed greywacke retaining some sedimentary features but with growth of metamorphic minerals.
• Metapelite — General term for metamorphosed fine-grained sedimentary rocks. Shows progressive mineral changes with increasing grade.
• Mica Schist — Strongly foliated metamorphic rock rich in mica minerals muscovite and/or biotite with visible crystal grains.
• Migmatite — Mixed rock showing interleaved light leucosome and dark melanosome layers, representing partial melting of metamorphic rock.
• Molasse — Thick sequence of coarse continental sediments conglomerate, sandstone deposited in foreland basins after mountain uplift.
• Monzonite — Coarse-grained igneous rock with roughly equal amounts of plagioclase and alkali feldspar, with little quartz.
• Mudstone — Fine-grained clastic rock similar to shale but lacking fissility. Breaks into blocky fragments rather than thin sheets.
• Mugearite — Moderately alkaline volcanic rock transitional between hawaiite and benmoreite in the alkaline basalt series.
• Mylonite — Fine-grained, banded rock formed by ductile deformation along fault zones at depth. Grain size reduction by dynamic recrystallization.
• Nepheline Syenite — Alkaline intrusive rock containing nepheline instead of quartz. Indicates silica-undersaturated magma composition.
• Norite — Variety of gabbro distinguished by having orthopyroxene hypersthene instead of clinopyroxene as the dominant pyroxene.
• Novaculite — Dense, hard, fine-grained siliceous rock composed of microcrystalline quartz. Famous as a natural whetstone.
• Obsidian — A natural volcanic glass formed when silica-rich lava cools very rapidly. Used by ancient peoples for tools and weapons due to its ability to form…
• Obsidian (Snowflake) — Black obsidian containing white spherulitic inclusions of cristobalite that create a snowflake pattern.
• Oil Shale — Fine-grained sedimentary rock containing kerogen solid organic matter that yields oil when heated.
• Oolitic Limestone — Limestone composed of spherical ooids 0.25-2mm formed by concentric precipitation of calcite around a nucleus.
• Orbicular Granite — Rare granite containing spheroidal structures orbicules with concentric shells of alternating minerals.
• Packstone — Grain-supported limestone with carbonate mud micrite between grains. More grains than wackestone.
• Pantellerite — Highly evolved peralkaline rhyolite with dark color despite felsic composition. Rich in sodium and iron.
• Pegmatite — Extremely coarse-grained igneous rock crystals 2.5 cm usually of granitic composition. Source of rare minerals and gems.
• Peridotite — Dense, dark ultramafic rock composed mainly of olivine. The dominant rock of the Earth upper mantle.
• Perlite — Volcanic glass containing 2-5% water that expands dramatically when heated. Widely used in construction and horticulture.
• Phonolite — Fine-grained alkaline volcanic rock containing nepheline or leucite. Rings when struck, hence the name Greek: phone = sound .
• Phosphorite — Sedimentary rock containing 15% phosphate minerals, primarily apatite. Major source of phosphorus for fertilizers.
• Phyllite — Fine-grained foliated metamorphic rock intermediate between slate and schist. Displays a characteristic silky sheen from microscopic mica.
• Pitchstone — Dark volcanic glass with a resinous to pitchy luster, containing more water than obsidian but less than perlite.
• Porcellanite — Dense, hard siliceous rock resembling unglazed porcelain. Intermediate between diatomite and chert in crystallinity.
• Porphyry — Igneous rock with large crystals phenocrysts set in a fine-grained groundmass. Can be of any composition.
• Pumice — Extremely vesicular volcanic glass so light it floats on water. Formed during explosive eruptions of silicic magma.
• Pyroxenite — Dark ultramafic rock composed primarily of pyroxene minerals 90% . Forms in layered intrusions.
• Quartzite (Meta-conglomerate) — Metamorphosed conglomerate where pebbles are deformed and stretched by tectonic forces, often difficult to distinguish from quartzite.
• Rapakivi Granite — Distinctive granite with large rounded alkali feldspar megacrysts mantled by oligoclase rapakivi texture .
• Red Sandstone — Sandstone colored red by hematite cement or grain coatings. Indicates oxidizing terrestrial depositional conditions.
• Reef Limestone — Massive limestone formed by framework-building organisms such as corals, sponges, and algae in reef environments.
• Rhyodacite — Fine-grained volcanic rock transitional between dacite and rhyolite in composition.
• Rhyolite — Light-colored, fine-grained volcanic rock with the same composition as granite. Often shows flow banding.
• Rock Salt — Sedimentary rock composed primarily of halite NaCl . Forms massive beds and salt domes of great economic importance.
• Rodingite — Calcium-rich metasomatic rock found as altered dikes within serpentinite bodies. Composed of calcium silicate minerals.
• Sandstone — A clastic sedimentary rock composed mainly of sand-sized grains, typically quartz. The color varies based on mineral content and cementing material.
• Schist — A medium-grade metamorphic rock characterized by visible, aligned mica flakes that give it a shiny, foliated appearance.
• Scoria — Dark, vesicular volcanic rock denser than pumice. Forms cinder cones and is ejected during basaltic eruptions.
• Serpentinite — Green, often slippery rock formed by hydration of ultramafic rocks. Contains serpentine group minerals.
• Shale — Fine-grained clastic sedimentary rock formed from compacted clay and silt. The most common sedimentary rock, showing fissility.
• Siltstone — Clastic sedimentary rock composed primarily of silt-sized particles 0.004-0.0625 mm . Intermediate between mudstone and sandstone.
• Skarn — Coarse-grained metamorphic rock formed by metasomatic replacement at the contact between igneous intrusions and carbonate rocks.
• Slate — A fine-grained metamorphic rock formed from shale. Its perfect cleavage allows it to be split into thin, flat sheets, making it ideal for roofing tiles…
• Soapstone — Soft metamorphic rock composed primarily of talc. Has a soapy feel and is easily carved. Used since antiquity.
• Spotted Slate — Slate containing dark spots porphyroblasts of cordierite or andalusite from contact metamorphism near an intrusion.
• Suevite — Impact breccia containing a mixture of shocked and unshocked rock fragments plus glass from meteorite impact events.
• Syenite — Coarse-grained intrusive rock similar to granite but with little or no quartz. Dominated by alkali feldspar.
• Taconite — Hard, flinty iron-bearing rock containing 25-30% iron, requiring beneficiation concentration before smelting.
• Tactite — Alternative name for skarn. Contact metasomatic rock rich in calcium and magnesium silicates formed at igneous-carbonate contacts.
• Tephrite — Fine-grained alkaline volcanic rock containing nepheline or leucite but no olivine. Related to basanite.
• Tiger Iron — Striking metamorphic rock composed of alternating bands of tiger eye quartz, red jasper, and silver hematite.
• Tillite — Lithified glacial till composed of unsorted sediment deposited directly by glacial ice. Contains fragments of all sizes.
• Tonalite — Quartz-bearing intermediate plutonic rock with plagioclase as the dominant feldspar. Part of the TTG suite.
• Trachyte — Fine-grained volcanic rock rich in alkali feldspar with little or no quartz. The extrusive equivalent of syenite.
• Travertine — Dense, banded calcium carbonate deposited from hot springs and mineral-rich waters. Popular building stone.
• Troctolite — Olivine-plagioclase rock lacking pyroxene. Sometimes called trout rock due to spotted appearance.
• Trondhjemite — Light-colored tonalite with very low mafic mineral content. Part of the TTG tonalite-trondhjemite-granodiorite suite.
• Tufa — Porous, spongy calcium carbonate deposited at ambient temperatures from calcium-rich spring water.
• Tuff — Consolidated rock composed of volcanic ash and fragments <2 mm ejected during explosive eruptions.
• Turbidite — Sedimentary rock deposited by turbidity currents in deep marine environments. Shows characteristic graded bedding Bouma sequence .
• Unakite — Distinctive pink and green metamorphic rock composed of pink orthoclase feldspar and green epidote with quartz.
• Volcanic Breccia — Rock composed of angular volcanic fragments 64 mm cemented by finer volcanic material.
• Wackestone — Mud-supported limestone containing 10% grains fossils, peloids floating in a carbonate mud micrite matrix.
• Wehrlite — Ultramafic rock composed of olivine and clinopyroxene without orthopyroxene. Found in layered intrusions.
• Whiteschist — Rare, pale-colored high-pressure metamorphic rock containing talc and kyanite. Indicates unusual bulk composition.