They are identical chemically—both are composed of carbon (C), but physically, they are very different. Minerals which have the same chemistry but different crystal structures are called polymorphs . When you look at graphite and diamond, it is hard to imagine that they are identical chemically, for they are so different physically. Graphite is opaque and metallic- to earthy-looking, while diamonds are transparent and brilliant. Another important physical difference is their hardness. The hardness of minerals is compared using the Mohs Hardness Scale, a relative scale numbered 1 (softest) to 10 (hardest). Graphite is very soft and has a hardness of 1 to 2 on this scale. Diamonds are the hardest known natural substance and have a hardness of 10. No other naturally occurring substance has a hardness of 10. The crystal structure of graphite yields physical properties that permit the use of graphite as a lubricant and as pencil lead. The gem and industrial properties of diamond, physical properties that we cherish and exploit, are also a result of diamond's crystal structure.