Some rocks according to their use or texture are classified into different types. Know more about Skarn Texture and Coal Texture. Types of Skarn are Endoskarns whereas the types of Coal are Peat, Lignite, Sub-Bituminous Coal, Bituminous Coal, Anthracite, Graphite.During historic times, some rocks depending on their hardness were used to create Pictographs and Petroglyphs. Rocks which are hard were generally not used. Pictographs are paintings or drawings that have been placed or created on the surface of rock. Skarn and Coal were not used for creating pictographs. Petroglyph is an image which is carved into a rock. These are images created by removing part of a rock surface by incising, picking, carving or abrading, as a form of rock art. Skarn and Coal were not used for creating petroglyphs.