Extrusive igneous rocks erupt onto the surface, where they cool quickly to form small crystals. Some cool so quickly that they form an amorphous glass. These rocks include andesite, basalt, obsidian, pumice, rhyolite, scoria, and tuff.

Pictures and brief descriptions of some common igneous rock types are shown on this page.

Diorite is a coarse-grained, intrusive igneous rock that contains a mixture of feldspar, pyroxene, hornblende, and sometimes quartz. The specimen shown above is about two inches (five centimeters) across.

Basalt is a fine-grained, dark-colored extrusive igneous rock composed mainly of plagioclase and pyroxene. The specimen shown is about two inches (five centimeters) across.

Chemical sedimentary rocks, such as rock salt, iron ore, chert, flint, some dolomites, and some limestones, form when dissolved materials precipitate from solution.

Organic sedimentary rocks such as coal, some dolomites, and some limestones, form from the accumulation of plant or animal debris.

Photos and brief descriptions of some common sedimentary rock types are shown on this page.

Coal is an organic sedimentary rock that forms mainly from plant debris. The plant debris usually accumulates in a swamp environment. Coal is combustible and is often mined for use as a fuel. The specimen shown above is about two inches (five centimeters) across.

Non-foliated metamorphic rocks such as hornfels, marble, quartzite, and novaculite do not have a layered or banded appearance. Pictures and brief descriptions of some common types of metamorphic rocks are shown on this page.

Gneiss is a foliated metamorphic rock that has a banded appearance and is made up of granular mineral grains. It typically contains abundant quartz or feldspar minerals. The specimen shown above is about two inches (five centimeters) across.