Diamonds are formed in the upper mantle of the Earth, around 100 miles below surface, by a combination of extremely high temperatures and pressures which enable the growth of diamond crystals from carbon. These crystals are then brought to the surface from the upper mantle by a volcanic eruption, when the cooling molten magma forms a diamond-bearing kimberlite or lamproite. These are the source of the majority of diamonds mined today but are increasingly rare with only circa 30 diamond mines of scale in operation today.