There are three main types of rocks: igneous, sedimentary, and metamorphic. Igneous rocks can be extrusive, with little to no crystals, or intrusive, with large interlocking crystals. Sedimentary rocks form from the lithification of sediments. Metamorphic rocks are formed from heat and pressure altering existing rocks, and can be formed through contact or regional metamorphism. Common metamorphic rocks include gneiss, slate, and quartzite. The rock cycle shows how rocks continuously change between these three types over geological time.