Rock Identification Made Easy

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, especially if the location of where the rock was found is unknown. To identify a rock, think like a geologist and examine its physical characteristics for clues. The following tips and tables contain characteristics that will help you identify the most common rocks on earth.

Rock Identification Tips

First, decide whether your rock is igneous, sedimentary or metamorphic.

  • Igneous rocks such as granite or lava are tough, frozen melts with little texture or layering. Rocks like these contain mostly black, white and/or gray minerals.
  • Sedimentary rocks such as limestone or shale are hardened sediment with sandy or clay-like layers (strata). They are usually brown to gray in color and may have fossils and water or wind marks.
  • Metamorphic rocks such as marble are tough, with straight or curved layers (foliation) of light and dark minerals. They come in various colors and often contain glittery mica.

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 ​without using a magnifier. Fine grains are smaller and usually cannot be identified without using a magnifier.
  • Hardness: This is measured with the Mohs scale and refers to the minerals contained within a rock. In simple terms, hard rock scratches glass and steel, usually signifying the minerals quartz or feldspar, which have a Mohs hardness of 6 or higher. Soft rock does not scratch steel but will scratch fingernails (Mohs scale of 3 to 5.5), while very soft rock won't even scratch fingernails (Mohs scale of 1 to 2). 

    Rock Identification Chart

    Once you've determined what type of rock you've got, look closely at its color and composition. This will help you identify it. 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

     

    Need more help?

    Still having trouble identifying your rock? 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