The following is a list of rock types recognized by petrologists. There is no agreed number of specific types of rocks. Any unique combination of chemical composition, mineralogy, grain size, texture, or other distinguishing characteristics can describe rock types. Additionally, different classification systems exist for each major type of rock. There are three major types of rock, sedimentary, metamorphic, and igneous. They are all identified by their texture, streak, and location, among other factors.