How to Identify the 3 Major Types of Rocks

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Updated on December 23, 2018

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 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.

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Igneous Rocks

Basaltic organs — Picavet / Getty Images

Igneous rock is created by volcanic activity, forming from magma and lava as they cool and harden. It is most often black, gray, or white, and often has a baked appearance.

Igneous rock may form crystalline structures as it cools, giving it a granular appearance; if no crystals form, the result will be natural glass. Examples of common igneous rock 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 type of rock 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.

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Sedimentary Rocks

Morning on the Li River with rock formations in the back — John Seaton Callahan / Getty Images

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

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Metamorphic Rocks

Marble quarry in Carrara — Angel Villalba / Getty Images

Metamorphic rock formation occurs when sedimentary or igneous rock becomes changed, or metamorphosed, by conditions underground.

The four main agents responsible for metamorphosing rock are heat, pressure, fluids, and strain, all capable of acting and interacting in an almost infinite variety of ways.

Most of the thousands of rare minerals known to science occur in metamorphic rock. Common examples of metamorphic rock 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 and is recognizable by the flakes of mica it contains.
Serpentinite: This green, scaly rock is formed beneath the ocean as sediment is transformed by heat and pressure.

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 cast from outer space which survive the trip to earth. Some meteorites contain rocky material in addition to elements such as iron and nickel, while others are comprised solely of mineral compounds.

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

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

Geodes are sedimentary or metamorphic rocks containing a hollow, mineral-filled interior such as quartz.

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

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Alden, Andrew. "How to Identify the 3 Major Types of Rocks." ThoughtCo, Feb. 16, 2021, thoughtco.com/rock-type-identification-4147694. Alden, Andrew. (2021, February 16). How to Identify the 3 Major Types of Rocks. Retrieved from https://www.thoughtco.com/rock-type-identification-4147694 Alden, Andrew. "How to Identify the 3 Major Types of Rocks." ThoughtCo. https://www.thoughtco.com/rock-type-identification-4147694 (accessed May 30, 2023).