A diamond is a mineral, not a rock. It is a solid, crystalline material that is made out of an orderly arrangement of carbon atoms. However, diamonds are often found in rocks called kimberlites, which slowly rise through the crust of the Earth over eons.
Diamond is one of the hardest substances known and can be cut and faceted into gemstones of unmatched brilliance. The reason for this is the immensely strong and regular bonds between the carbon atoms. Most diamonds occur as well-formed crystals that are not greatly damaged by their transport through the Earth's crust. Most diamond crystals appear as octahedra.