How did diamonds reach the Earths surface?
Diamonds are accidentally swept to the surface and near-surface during highly explosive volcanic eruptions of deep-origin. Diamond-bearing volcanic craters and pyroclastic deposits are underlain by volcanic pipes (diatremes) or by intrusive sills and dikes. The diamond-bearing craters and volcanic pipes are often less than 1 hectare (10,000 m2) in area and normally less than 20 hectares. The magma for such diamond-bearing volcanic eruptions must originate at the profound depths where diamonds are formed, i.e. usually 150 km to 250 km (but rarely as deep as 600 km) below surface. Such volcanic events have been relatively rare throughout geological time, for it is seldom that faults fracture so deep into the lower lithosphere and upper mantle to tap partly molten rocks. As diamonds are unstable at normal surface temperatures and pressures (given an opportunity they will slowly revert to the stable carbon polymorph graphite) they have to be rapidly brought to the surface by erupting magma
