A diamond is a solid form of the element carbon with its atoms organized in a diamond cubic crystal structure. Another solid form of carbon known as graphite is the chemically stable form of carbon at ambient temperature and pressure, although diamond nearly never transforms to it. Diamond is a hard material; graphite is very soft. Pure diamond is colorless, but colored diamonds are available if they contain impurities such as brown coal or clay. The chemical formula for diamond is C 12, which means that each atom of carbon is bonded to three other carbon atoms. In natural diamonds, there are also small amounts of other elements such as oxygen or nitrogen. These impurities have little effect on the physical properties of the diamond, but they do change its chemical behavior. For example, the presence of oxygen makes diamond less resistant to oxidation than pure carbon. Diamond has many useful properties: it is the hardest substance known to man, it is transparent to light up to about 950 nm, it produces no toxic fumes when burned, and it is biocompatible. These properties make diamond useful for many applications including cutting tools, wear-resistant products, and optical devices. It is also used as an electrical contact material because it has low resistance compared to other common materials.