Rock Identification Tables

These tables will help you identify almost any rock 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.

• Igneous: A tough, frozen melt with little texture or layering; mostly black, white and/or gray minerals; may look like lava (about igneous rocks)
• Sedimentary: Hardened sediment with layers (strata) of sandy or clayey stone; mostly brown to gray; may have fossils and water or wind marks (about sedimentary rocks)
• Metamorphic: Tough rock with layers (foliation) of light and dark minerals, often curved; various colors; often glittery from mica (about metamorphic rocks)

Next, check the rock's grain size and hardness. Then start in the left column of the appropriate table below and work your way across. Follow the links to pictures and more information. If you don't find a match, try another of the three big types.

Grain Size: "Coarse" grains are visible to the naked eye (greater than about 0.1 millimeter), 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.

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	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	foliated	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	Hardness	Usual Color	Other	Rock Type
foliated	fine	soft	dark	"tink" when struck	Slate
foliated	fine	soft	dark	shiny; crinkly foliation	Phyllite
foliated	coarse	hard	mixed dark and light	wrinkled foliation; often has large crystals	Schist
foliated	coarse	hard	mixed	banded	Gneiss
foliated	coarse	hard	mixed	distorted "melted" layers	Migmatite
foliated	coarse	hard	dark	mostly hornblende	Amphibolite
nonfoliated	fine	soft	greenish	shiny, mottled surface	Serpentinite
nonfoliated	fine or coarse	hard	dark	dull and opaque colors, found near intrusions	Hornfels
nonfoliated	coarse	hard	red and green	dense; garnet and pyroxene	Eclogite
nonfoliated	coarse	soft	light	calcite or dolomite by the acid test	Marble
nonfoliated	coarse	hard	light	quartz (no fizzing with acid)	Quartzite

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