There are three types of rocks, namely Metamorphic, Igneous, and Sedimentary. Sedimentary rocks are formed due to hardening of sediments, thus forming soft and easily-crumbled rocks, i.e. Limestone . Igneous rocks are basically formed because of the cooling and hardening of magma, as a result they turn into rocks which are eventually glass-like and shiny(Obsidian is the best example). Metamorphic rocks, in the mean time, are formed due to the changes because of heat and pressure under the earth's surface. Marble is an example of a metamorphic rock.