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Rock Identification Tables

Identify almost any rock type you're likely to find

By , About.com Guide

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Start by getting an idea of your basic rock class—igneous, sedimentary or metamorphic. Next observe the rock's texture—its grain size and overall pattern—conduct a basic hardness test, and identify the minerals that compose it as well as you can. With that information, you can consult the table for the appropriate rock class. Detailed instructions are beneath the tables.

Identification of Igneous Rocks
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% olivine Dunite
very coarse any color usually in small intrusive bodies typically granitic Pegmatite

Identification of Sedimentary Rocks
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

Identification of Metamorphic Rocks
Foliation Grain Size Usual Color Other Rock Type
foliated fine light very soft; greasy feel Soapstone
foliated fine dark soft; "tink" when struck Slate
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

These three tables will help you identify almost any rock type you're likely to find. Read How to Look at a Rock for help with your observations. 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; may look like granite or like lava (about igneous rocks)
  • Sedimentary rocks are hardened sediment with sandy or clayey layers (strata); mostly brown to gray; 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; various colors; 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. (using a magnifier, 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 6-7 and up); "soft" rock does not scratch a steel knife but scratches fingernails (Mohs 3-5.5); "very soft" rock does not scratch fingernails (Mohs 1-2). 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. If you don't find a match, try another table.

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