How to Identify the 3 Major Types of Rocks

In geology, pictures of rocks can be used to help you best determine which of the three major types a particular rock belongs to: igneous, sedimentary, or metamorphic.

By comparing your rock sample with photographic examples, you can identify key characteristics such as how the rock was formed, what minerals and other materials it contains, and where the rock may have come from.

Sooner or later, you're bound to encounter hard, rock-like substances that aren't actually rocks. Such items include man-made substances like concrete and bricks, as well as rocks from outer space (such as meteorites) that have dubious origins.

Before beginning the identification process, make sure that your sample has been washed to remove dirt. You'll also want to make sure you have a freshly cut surface so you can identify color, grain structure, stratification, texture, and other characteristics.

Basaltic organs
Picavet / Getty Images

Igneous rocks are created by volcanic activity and form as magma and lava cool and harden. They are most often black, gray, or white in color, and often have a baked appearance. As they cool, these rocks may form crystalline structures, giving them a granular appearance; if no crystals form, the result will be natural glass. Examples of common igneous rocks include:

Basalt: Formed from low-silica lava, basalt is the most common type of volcanic rock. It has a fine grain structure and is usually black to gray in color.

Granite: This igneous rock may range from white to pink to gray, depending on the mix of quartz, feldspar, and other minerals it contains. It is among the most abundant rocks on the planet.

Obsidian: This is formed when high-silica lava cools rapidly, forming volcanic glass. It is usually glossy black in color, hard, and brittle. More »

Morning on the Li River
John Seaton Callahan / Getty Images

Sedimentary rocks, also called stratified rocks, are formed over time by wind, rain, and glacial formations. They may be formed by erosion, compression, or dissolution. Sedimentary rocks may range from green to gray, or red to brown, depending on their iron content, and are usually softer than igneous rocks. Examples of common sedimentary rocks include:

Bauxite: Usually found at or near the earth's surface, this sedimentary rock is used in the production of aluminum. It ranges from red to brown with a large grain structure.

Limestone: Formed by dissolved calcite, this grainy rock often contains fossils from the ocean because it is formed by layers of dead coral and other marine creatures. It ranges from cream to gray to green in color.

Halite: More commonly known as rock salt, this sedimentary rock is formed from dissolved sodium chloride, which forms large crystals. More »

Carrara. Marble quarry
Angel Villalba / Getty Images

Metamorphic rock formations occur when sedimentary and igneous rocks become changed, or metamorphosed, by conditions underground. The four main agents that metamorphose rocks are heat, pressure, fluids, and strain. These agents can act and interact in an almost infinite variety of ways. Most of the thousands of rare minerals known to science occur in metamorphic rocks. Common examples of metamorphic rocks include: 

Marble: This coarse-grained, metamorphosed limestone ranges in color from white to gray to pink. The colored bands (called veins) that give marble its characteristic swirled appearance are caused by mineral impurities.

Phyllite: This shiny, colorful metamorphosed slate ranges in color from black to green-gray. It can be recognized by the flakes of mica that it contains.

Serpentinite: This green, scaly rock is formed beneath the ocean as sediment is transformed by heat and pressure.  More »

Other Rocks and Rock-Like Objects

Just because a sample looks like a rock doesn't mean it is one, however. Here are a few of the most common that geologists encounter:

Meteorites are (usually) small, rock-like formations originally from outer space that survived the trip to earth. Some meteorites contain rocky material in addition to elements like iron and nickel, while others are comprised solely of mineral compounds.

Concretions resemble smooth, often oblong masses found along a riverbed, apparently cemented together. These are not rocks, but masses formed by dirt, minerals, and other water-borne debris.

Fulgurites are hard, jagged, oblong masses are formed from soil, rock, and/or sand that has been fused together by a lightning strike.

Geodes are sedimentary or metamorphic rocks that contain a hollow, mineral-filled interior like quartz.

Thundereggs are solid, agate-filled lumps found in volcanic regions. They resemble geodes with opened.

Nearly 30 US states have official state rocks, ranging from marble in Alabama to red granite in Wisconsin.

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Alden, Andrew. "How to Identify the 3 Major Types of Rocks." ThoughtCo, Feb. 20, 2018, Alden, Andrew. (2018, February 20). How to Identify the 3 Major Types of Rocks. Retrieved from Alden, Andrew. "How to Identify the 3 Major Types of Rocks." ThoughtCo. (accessed March 18, 2018).