Diamond, composed of pure carbon, is the hardest mineral known, and also the most popular gemstone. It occurs in a unique rock known as kimberlite that was created at great depths and high temperature and pressure. Diamond also occurs in streams and offshore deposits, when the crystals have weathered out of their host rocks and transported by water. The most characteristic shape of a diamond crystal is the octahedron. Such crystals are usually small and scattered within their host rock. Matrix specimens, such as this one from South African, are very rare because diamond recovery is a large scale mass-production operation and the crushed rock is processed by machines.