Rocks primarily derive their color from their constituent minerals. Rocks rich in feldspar and quartz are usually light in color, whereas those that have a lot of pyroxenes, olivines or amphiboles are dark in color. However, the size of the particles that make up the rock can affect the color as well. For example, feldspar and granite rocks may be light or dark, depending on how they formed.
Color is a useful criterion to consider when identifying unknown rocks. While color alone does not often provide a positive identification, it is helpful when considered along with the unknown rock’s hardness, geographic origin or structure. The exterior colors of rocks can become altered by erosion, so it is best to break open a rock and observe the color of the inside. It may also be helpful to observe the color while the rock is both wet and dry.
Aside from erosion, a process called chemical weathering may also cause the color of rocks to change. For example, rocks containing iron tend to rust when they come into contact with oxygen or water. When they begin to rust, they turn reddish-brown.
Regardless of their color and chemical composition, all earthly rocks are formed in one of three ways. Sedimentary rocks are formed from compressed sand and dust, while igneous rocks are volcanic in origin and are composed of cooled magma. Metamorphic rocks are named because they changed states in the past due to high temperature and pressure.