Field identification library
Identify common rocks by combining texture, grain size, minerals, structures, and simple field tests. Each isolated guide explains lookalikes and the limits of hand-sample identification.
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A reliable sequence
First decide whether grains interlock, are cemented, or align in a metamorphic fabric. Then compare grain size, composition, bedding, vesicles, cleavage, hardness, and acid reaction.
Learn the field-test proceduresNeed help narrowing the possibilities? Use the interactive identifier. For individual grains, continue to the mineral identification guides.
Rock family
Intrusive igneous rockGranite is a coarse-grained intrusive igneous rock usually recognized by interlocking light-colored feldspar and quartz with scattered dark mica or amphibole.
Extrusive igneous rockBasalt is a dark, fine-grained volcanic rock formed from relatively iron- and magnesium-rich lava; crystals are commonly too small to distinguish by eye.
Felsic extrusive igneous rockRhyolite is a silica-rich volcanic rock, commonly light colored and fine grained, that is broadly comparable in composition to granite but cooled at or near the surface.
Volcanic glassObsidian is natural volcanic glass produced when silica-rich melt cools so quickly that an orderly crystal framework does not develop.
Intermediate intrusive igneous rockDiorite is a coarse-grained intrusive igneous rock, commonly recognized by an interlocking salt-and-pepper mixture of pale plagioclase and dark amphibole, pyroxene, or biotite.
Mafic intrusive igneous rockGabbro is a dark, coarse-grained intrusive igneous rock made chiefly of calcium-rich plagioclase and pyroxene, commonly with olivine or iron-titanium oxides.
Intermediate extrusive igneous rockAndesite is a fine-grained intermediate volcanic rock, commonly medium gray and frequently porphyritic with visible plagioclase crystals in a much finer groundmass.
Rock family
Clastic sedimentary rockSandstone is made of sand-size grains cemented or compacted together; grain texture, bedding, and cement distinguish it from crystalline rocks.
Carbonate sedimentary rockLimestone is a sedimentary rock composed mostly of calcium carbonate, commonly as calcite derived from shells, lime mud, chemical precipitation, or later recrystallization.
Fine-grained clastic sedimentary rockShale is a very fine-grained sedimentary rock distinguished by its tendency to split into thin layers along closely spaced bedding-related planes.
Coarse clastic sedimentary rockConglomerate contains rounded gravel-size clasts held in a finer matrix or cement, recording transport energetic enough to move pebbles, cobbles, or boulders.
Dolomite-rich sedimentary rockDolostone is a sedimentary carbonate rock composed mainly of the mineral dolomite, commonly distinguished from limestone by its weaker acid response, slightly greater hardness, and frequent crystalline or sugary texture.
Fine-grained clastic sedimentary rockSiltstone is a clastic sedimentary rock made mostly of silt-size particles, finer than sandstone but generally coarser and grittier than clay-rich mudstone.
Fine-grained clastic sedimentary rockMudstone is a lithified mixture dominated by clay- and silt-size particles that lacks the strong fissility used to define shale.
Rock family
Nonfoliated metamorphic rockMarble is recrystallized carbonate rock, usually derived from limestone or dolostone, with interlocking calcite or dolomite crystals and little or no foliation.
Fine-grained foliated metamorphic rockSlate is a fine-grained metamorphic rock that splits along closely spaced, planar slaty cleavage, commonly formed from shale or mudstone at relatively low metamorphic grade.
Nonfoliated metamorphic rockQuartzite is a hard metamorphic rock formed when quartz-rich sandstone recrystallizes into a tightly interlocking mosaic of quartz grains.
Coarse-grained foliated metamorphic rockGneiss is a high-grade metamorphic rock identified by segregation of light and dark minerals into bands or lenses rather than by one fixed mineral composition.
Medium- to coarse-grained foliated metamorphic rockSchist is a foliated metamorphic rock with enough aligned platy or elongate minerals—commonly mica—to produce a visible sheen and a tendency to split along schistosity.
Fine-grained foliated metamorphic rockPhyllite is a fine-grained foliated metamorphic rock between slate and schist, recognized by a silky sheen and gently wavy foliation produced by microscopic mica.
Amphibole-rich metamorphic rockAmphibolite is a dark metamorphic rock composed mainly of amphibole and plagioclase, commonly medium to coarse grained and variably foliated.
Common lookalikes
Start with the question you are trying to resolve. Each isolated comparison page focuses on the observations that separate two commonly confused rocks.
