Science, Tech, Math › Science Rock Identification Made Easy Share Flipboard Email Print WIN-Initiative / Getty Images Science Geology Types Of Rocks Landforms and Geologic Features Geologic Processes Plate Tectonics Chemistry Biology Physics Astronomy Weather & Climate Table of Contents Expand Rock Identification Tips Rock Identification Chart Igneous Rock Identification Sedimentary Rock Identification Metamorphic Rock Identification Need More Help? By Andrew Alden Geology Expert B.A., Earth Sciences, University of New Hampshire Andrew Alden is a geologist based in Oakland, California. He works as a research guide for the U.S. Geological Survey. our editorial process Andrew Alden Updated February 24, 2020 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 orsoft 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. How to Look at a Rock Like a Geologist How to Identify the 3 Major Types of Rocks How to Perform the Mohs Test Everything You Need to Know About Igneous Rocks Minerals of the Earth's Surface Understanding Commercial Granite The 12 Most Common Blue, Violet, and Purple Minerals Guide to Identifying Yellow Minerals What Hornfels Is and How It Forms Quartzite Rock Geology and Uses A Few Rocks That Include Silicate Materials Properties of Metamorphic Rocks Carbonate Minerals Mohs Scale of Mineral Hardness Identifying Black Minerals What Is Sandstone?