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Rocks are everywhere. Huge plates of rock lie under both the ground we walk on and the ocean floor. Mountains are giant blocks of rock. Sand is made of many tiny bits of rock. Earth has many kinds of rocks. Scientists separate these rocks into three groups. They are igneous rocks, metamorphic rocks, and sedimentary rocks.

Rocks are solid matter made of minerals. Minerals are nonliving, naturally occurring matter. Most rocks, such as gabbro, are made up of two or more minerals. Some rocks, such as quartzite, have only one mineral. Rocks come in many shapes, but each mineral forms crystals in its own special shape. For example, the mineral beryl always forms a shape with six sides.