Some rocks according to their use or texture are classified into different types. Know more about Hornfels Texture and Coal Texture. Types of Hornfels are Biotite hornfels 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. Hornfels was used to create pictographs whereas Coal is not used to create 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. Hornfels was used to create petroglyphs whereas Coal was not used to create petroglyphs.