Rock Identification Tables

An Easy Way to Identify Igneous, Sedimentary and Metamorphic Rocks

Chert
Chert is just one possibility. Photo by Andrew Alden

Any good rockhound is bound to come across a rock that he or she has trouble identifying. If you know the location of where the rock was found, this handy interactive rock identification tool from the USGS is a good place to start. It can really narrow down your possibilities, and it gives a great look at the mineral and rock resources of an area (make sure to check the "Geologic Map" option). 

If you don't know where your rock was found, then identifying it might be a little tougher.

The tables at the end of this article give you an easy-to-use method for identifying rocks based on common observations (read How to Look at a Rock for help on what you see).

Rock Identification Tips

First, decide whether your rock is igneous, sedimentary or metamorphic. This is not that hard!

  • Igneous rocks are tough, frozen melts with little texture or layering; mostly black, white and/or gray minerals. They may look like granite or like lava (about igneous rocks).
  • Sedimentary rocks are hardened sediment with sandy or clay-like layers (strata); mostly brown to gray. They may have fossils and water or wind marks (about sedimentary rocks).
  • Metamorphic rocks are tough, with straight or curved layers (foliation) of light and dark minerals; they come in various colors. They are often glittery with mica (about metamorphic rocks).

Next, check the rock's grain size and hardness.

  • Grain Size: "Coarse" grains are visible to the naked eye, and the minerals can usually be identified using a magnifier; "fine" grains are smaller and usually cannot be identified with a magnifier. ( Get tips on using a magnifier and identifying minerals.)
  • Hardness: Hardness (as measured with the Mohs scale) actually refers to minerals rather than rocks, so a rock may be crumbly yet consist of hard minerals. But in simple terms, "hard" rock scratches glass and steel, usually signifying the minerals quartz or feldspar (Mohs hardness 6 to 7 and up); "soft" rock does not scratch a steel knife but scratches fingernails (Mohs 3 to 5.5); "very soft" rock does not scratch fingernails (Mohs 1 to 2). Igneous rocks are always hard. Metamorphic rocks are generally hard.

    Now start in the left column of the appropriate table and work your way across. Follow the links to pictures and more information. 

    Igneous Rock Identification

    Grain SizeUsual ColorOtherCompositionRock Type
    finedarkglassy appearancelava glassObsidian
    finelightmany small bubbleslava froth from sticky lavaPumice
    finedarkmany large bubbleslava froth from fluid lavaScoria
    fine or mixedlightcontains quartzhigh-silica lavaFelsite
    fine or mixedmediumbetween felsite and basaltmedium-silica lavaAndesite
    fine or mixeddarkhas no quartzlow-silica lavaBasalt
    mixedany colorlarge grains in fine-grained matrixlarge grains of feldspar, quartz, pyroxene or olivinePorphyry
    coarselightwide range of color and grain sizefeldspar and quartz with minor mica, amphibole or pyroxeneGranite
    coarselightlike granite but without quartzfeldspar with minor mica, amphibole or pyroxeneSyenite
    coarselight to mediumlittle or no alkali feldsparplagioclase and quartz with dark mineralsTonalite
    coarsemedium to darklittle or no quartzlow-calcium plagioclase and dark mineralsDiorite
    coarsemedium to darkno quartz; may have olivinehigh-calcium plagioclase and dark mineralsGabbro
    coarsedarkdense; always has olivineolivine with amphibole and/or pyroxenePeridotite
    coarsedarkdensemostly pyroxene with olivine and amphibolePyroxenite
    coarsegreendenseat least 90% olivineDunite
    very coarseany colorusually in small intrusive bodiestypically graniticPegmatite

     

    Sedimentary Rock Identification

    HardnessGrain SizeCompositionOtherRock Type
    hardcoarseclean quartzwhite to brownSandstone
    hardcoarsequartz and feldsparusually very coarseArkose
    hard or softmixedmixed sediment with rock grains and claygray or dark and "dirty"Wacke/
    Graywacke
    hard or softmixedmixed rocks and sedimentround rocks in finer sediment matrixConglomerate
    hard or
    soft
    mixedmixed rocks and sedimentsharp pieces in finer sediment matrixBreccia
    hardfinevery fine sand; no clayfeels gritty on teethSiltstone
    hardfinechalcedonyno fizzing with acidChert
    softfineclay mineralssplits in layersShale
    softfinecarbonblack; burns with tarry smokeCoal
    softfinecalcitefizzes with acidLimestone
    softcoarse or finedolomiteno fizzing with acid unless powderedDolomite rock
    softcoarsefossil shellsmostly piecesCoquina
    very softcoarsehalitesalt tasteRock Salt
    very softcoarsegypsumwhite, tan or pinkRock Gypsum

     

    Metamorphic Rock Identification

    FoliationGrain SizeUsual ColorOtherRock Type
    foliatedfinelightvery soft; greasy feelSoapstone
    foliatedfinedarksoft; strong cleavageSlate
    nonfoliatedfinedarksoft; massive structureArgillite
    foliatedfinedarkshiny; crinkly foliationPhyllite
    foliatedcoarsemixed dark and lightcrushed and stretched fabric; deformed large crystalsMylonite
    foliatedcoarsemixed dark and lightwrinkled foliation; often has large crystalsSchist
    foliatedcoarsemixedbandedGneiss
    foliatedcoarsemixeddistorted "melted" layersMigmatite
    foliatedcoarsedarkmostly hornblendeAmphibolite
    nonfoliatedfinegreenishsoft; shiny, mottled surfaceSerpentinite
    nonfoliatedfine or coarsedarkdull and opaque colors, found near intrusionsHornfels
    nonfoliatedcoarsered and greendense; garnet and pyroxeneEclogite
    nonfoliatedcoarselightsoft; calcite or dolomite by the acid testMarble
    nonfoliatedcoarselightquartz (no fizzing with acid)Quartzite

     

    Still having trouble? Try contacting a geologist from a local natural history museum or university. The response may take a week or more, but it's effective to get your question answered by an expert! 

    Edited by Brooks Mitchell