The 3 main categories of rocks are igneous, sedimentary and metamorphic. Igneous rocks are made of cooled lava. Granite is an igneous rock. Sedimentary rocks are made of sand, clay, skeletons, shells, etc. Sandstone and Limestone are examples of sedimentary rocks. Metamorphic rock is rock that has been changed by heat and pressure. Marble is an example of metamorphic rock.