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 has 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 Typefinedarkglassy appearancelava glassObsidianfinelightmany small bubbleslava froth from sticky lavaPumicefinedarkmany large bubbleslava froth from fluid lavaScoriafine or mixedlightcontains quartzhigh-silica lavaFelsitefine or mixedmediumbetween felsite and basaltmedium-silica lavaAndesitefine or mixeddarkhas no quartzlow-silica lavaBasaltmixedany colorlarge grains in fine-grained matrixlarge grains of feldspar, quartz, pyroxene or olivinePorphyrycoarselightwide range of color and grain sizefeldspar and quartz with minor mica, amphibole or pyroxeneGranitecoarselightlike granite but without quartzfeldspar with minor mica, amphibole or pyroxeneSyenitecoarselight to mediumlittle or no alkali feldsparplagioclase and quartz with dark mineralsTonalitecoarsemedium to darklittle or no quartzlow-calcium plagioclase and dark mineralsDioritecoarsemedium to darkno quartz; may have olivinehigh-calcium plagioclase and dark mineralsGabbrocoarsedarkdense; always has olivineolivine with amphibole and/or pyroxenePeridotitecoarsedarkdensemostly pyroxene with olivine and amphibolePyroxenitecoarsegreendenseat least 90 percent olivineDunitevery coarseany colorusually in small intrusive bodiestypically graniticPegmatite
Sedimentary Rock Identification
HardnessGrain SizeCompositionOtherRock Typehardcoarseclean quartzwhite to brownSandstonehardcoarsequartz and feldsparusually very coarseArkosehard or softmixedmixed sediment with rock grains and claygray or dark and "dirty"Wacke/ Graywackehard or softmixedmixed rocks and sedimentround rocks in finer sediment matrixConglomeratehard or softmixedmixed rocks and sedimentsharp pieces in finer sediment matrixBrecciahardfinevery fine sand; no clayfeels gritty on teethSiltstonehardfinechalcedonyno fizzing with acidChertsoftfineclay mineralssplits in layersShalesoftfinecarbonblack; burns with tarry smokeCoalsoftfinecalcitefizzes with acidLimestonesoftcoarse or finedolomiteno fizzing with acid unless powderedDolomite rocksoftcoarsefossil shellsmostly piecesCoquinavery softcoarsehalitesalt tasteRock Saltvery softcoarsegypsumwhite, tan or pinkRock Gypsum
Metamorphic Rock Identification
FoliationGrain SizeUsual ColorOtherRock Typefoliatedfinelightvery soft; greasy feelSoapstonefoliatedfinedarksoft; strong cleavageSlatenonfoliatedfinedarksoft; massive structureArgillitefoliatedfinedarkshiny; crinkly foliationPhyllitefoliatedcoarsemixed dark and lightcrushed and stretched fabric; deformed large crystalsMylonitefoliatedcoarsemixed dark and lightwrinkled foliation; often has large crystalsSchistfoliatedcoarsemixedbandedGneissfoliatedcoarsemixeddistorted "melted" layersMigmatitefoliatedcoarsedarkmostly hornblendeAmphibolitenonfoliatedfinegreenishsoft; shiny, mottled surfaceSerpentinitenonfoliatedfine or coarsedarkdull
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