Rocks are classified by according to their mineral and chemical composition. There are three classifications for rocks: igneous, sedimentary and metamorphic.
Igneous rocks are formed by crystallization. The source of the crystallization occurs thanks to magma and is often called by the common name of lava rocks. Sedimentary rocks are created from sediment that has accumulated on the surface through various means, such as weathering or erosion. Metamorphic rocks can be both igneous or sedimentary. These types of rocks are formed when the first two types are put under great pressure, high temperature or sometimes both. These changes normally occur deep below the surface.