Diamond, Graphite, and Graphene are well-known allotropes of carbon with different crystal structures and physical properties. For example, diamond is very hard with a tetrahedral structure, graphite is soft with a layered structure, and graphene is a single layer of carbon atoms in a hexagonal lattice. Coal, however, is a naturally occurring fossil fuel composed primarily of carbon but also containing varying amounts of hydrogen, oxygen, nitrogen, sulfur, and other elements, as well as mineral matter. It is a complex mixture, not a pure element existing in a specific crystalline or amorphous form.