It is helpful to have a basic understanding of the types of rocks out there when incorporating rocks into backyard landscapes. There are three types of rocks: sedimentary, metamorphic, and igneous rocks. Sedimentary rock, as the name implies, is formed from sediments or layers of sand, shells, pebbles, or other hard substances. Sedimentary rock tends to be somewhat soft and may break easily. An example of sedimentary rock is limestone. Metamorphic rock is that which is formed under great heat and pressure under the earth's surface.