There are many different kinds of rocks. But rocks have many things in common, too. All rocks are solid, are made naturally, and are made up of groups of smaller particles and minerals that are stuck together. There are three types of rocks: igneous, sedimentary, and metamorphic. Rocks have been used to make buildings, walls, and roads. In the form of fossils, rocks have taught us many things about the kinds of plants and animals that have lived on Earth.