Flexi Says: A rock is a naturally formed, non-living Earth material. Rocks are made of minerals. Rocks are named for the combinations of minerals they are made of and the ways those minerals came together. Remember that different minerals form under different environmental conditions. So the minerals in a rock contain clues about the conditions in which the rock formed. Geologists group rocks based on how they form. There are three major rock types. Igneous rocks form when magma cools below Earth’s surface or lava cools at the surface. Sedimentary rocks form when sediments are compacted and cemented together. Sediments are pieces of rock. They may be gravel, sand, silt, or clay. Some sedimentary rocks form the solid minerals left behind after a liquid evaporates. Metamorphic rocks form when an existing rock is changed by heat or pressure. The minerals in the rock change but do not melt. The rock experiences these changes within the Earth.