Rock types

How each common rock is defined, with photos and thin-section links. Thin sections often settle what a hand sample can't: the minerals, textures and fossils inside.

Igneous

Named by composition (felsic to ultramafic) and grain size (intrusive and coarse, or extrusive and fine).

no open-licensed photoGraniteDioriteGabbroPeridotite and duniteRhyoliteAndesiteBasaltno open-licensed photoKomatiiteno open-licensed photoObsidian, pumice and scoria

Sedimentary

Clastic rocks are named by grain size and composition; chemical, biochemical and organic rocks by what they're made of.

no open-licensed photoConglomerate and brecciaQuartz areniteArkoseno open-licensed photoLithic areniteno open-licensed photoWacke (greywacke)no open-licensed photoMudrocks: shale, mudstone, siltstone, claystoneLimestoneno open-licensed photoChalkno open-licensed photoCoquinaOoliteTravertine and tufano open-licensed photoDolostoneChertno open-licensed photoDiatomiteno open-licensed photoRadiolariteEvaporites: rock salt and gypsumBanded iron formationCoal: peat to anthraciteno open-licensed photoOil shale

Metamorphic

Foliated rocks get coarser from slate to gneiss as grade increases; non-foliated rocks are named by mineralogy.

SlatePhylliteSchistGneissno open-licensed photoAmphiboliteno open-licensed photoMarbleQuartziteno open-licensed photoHornfelsMigmatite

Limestone classification Fossils in thin section Textures