Rock Identification Made Easy

View of Badlands formations
WIN-Initiative / Getty Images

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 Size Usual Color Other Composition Rock Type
fine dark glassy appearance lava glass Obsidian
fine light many small bubbles lava froth from sticky lava Pumice
fine dark many large bubbles lava froth from fluid lava Scoria
fine or mixed light contains quartz high-silica lava Felsite
fine or mixed medium between felsite and basalt medium-silica lava Andesite
fine or mixed dark has no quartz low-silica lava Basalt
mixed any color large grains in fine-grained matrix large grains of feldspar, quartz, pyroxene or olivine Porphyry
coarse light wide range of color and grain size feldspar and quartz with minor mica, amphibole or pyroxene Granite
coarse light like granite but without quartz feldspar with minor mica, amphibole or pyroxene Syenite
coarse light to medium little or no alkali feldspar plagioclase and quartz with dark minerals Tonalite
coarse medium to dark little or no quartz low-calcium plagioclase and dark minerals Diorite
coarse medium to dark no quartz; may have olivine high-calcium plagioclase and dark minerals Gabbro
coarse dark dense; always has olivine olivine with amphibole and/or pyroxene Peridotite
coarse dark dense mostly pyroxene with olivine and amphibole Pyroxenite
coarse green dense at least 90 percent olivine Dunite
very coarse any color usually in small intrusive bodies typically granitic Pegmatite

 

Sedimentary Rock Identification

Hardness Grain Size Composition Other Rock Type
hard coarse clean quartz white to brown Sandstone
hard coarse quartz and feldspar usually very coarse Arkose
hard or soft mixed mixed sediment with rock grains and clay gray or dark and "dirty" Wacke/
Graywacke
hard or soft mixed mixed rocks and sediment round rocks in finer sediment matrix Conglomerate
hard or
soft
mixed mixed rocks and sediment sharp pieces in finer sediment matrix Breccia
hard fine very fine sand; no clay feels gritty on teeth Siltstone
hard fine chalcedony no fizzing with acid Chert
soft fine clay minerals splits in layers Shale
soft fine carbon black; burns with tarry smoke Coal
soft fine calcite fizzes with acid Limestone
soft coarse or fine dolomite no fizzing with acid unless powdered Dolomite rock
soft coarse fossil shells mostly pieces Coquina
very soft coarse halite salt taste Rock Salt
very soft coarse gypsum white, tan or pink Rock Gypsum

Metamorphic Rock Identification

Foliation Grain Size Usual Color Other Rock Type
foliated fine light very soft; greasy feel Soapstone
foliated fine dark soft; strong cleavage Slate
nonfoliated fine dark soft; massive structure Argillite
foliated fine dark shiny; crinkly foliation Phyllite
foliated coarse mixed dark and light crushed and stretched fabric; deformed large crystals Mylonite
foliated coarse mixed dark and light wrinkled foliation; often has large crystals Schist
foliated coarse mixed banded Gneiss
foliated coarse mixed distorted "melted" layers Migmatite
foliated coarse dark mostly hornblende Amphibolite
nonfoliated fine greenish soft; shiny, mottled surface Serpentinite
nonfoliated fine or coarse dark dull and opaque colors, found near intrusions Hornfels
nonfoliated coarse red and green dense; garnet and pyroxene Eclogite
nonfoliated coarse light soft; calcite or dolomite by the acid test Marble
nonfoliated coarse light quartz (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. It's more effective to get your question answered by an expert.