Diamond. This is a 59 carat un-cut (rough) diamond. The carat is a unit of mass equal to 200 milligrams. Diamonds are a hard crystalline form of carbon that are used in jewellery, and in industry as a material for cutting tools. They occur naturally in rough (un-cut) forms. These raw diamonds are then cut into different shapes and polished, to give different effects when mounted in jewellery.