Diamonds are brought to the surface from the mantle in a rare type of magma called kimberlite and erupted at a rare type of volcanic vent called a diatreme or pipe. Kimberlite is a gas-rich, potassic ultramafic igneous rock that contains the minerals olivine, phlogopite, diopside, serpentine, calcite, and minor amounts of apatite, magnetite, chromite, garnet, diamond, and other upper mantle minerals. Upper mantle xenoliths are found in some kimberlite and provide clues to the magma's origin. The source depth for kimberlite magmas is estimated at 200 km, more than twice as deep as the source region for most magmas. At a depth of 200 km the pressure is 60,000 times greater than the surface and the temperature is about 1500 C. Kimberlite magmas are rich in carbon dioxide and water which brings the magma quickly and violently to the surface.
Most kimberlites occur as multiple intrusive events.
Kimberlite was named for the rock associated with diamonds in Kimberley, South Africa. A tuff cone is at the surface and formed by base-surge deposits. In the subsurface, a funnel-shaped body narrows to a depth of hundreds of meters. Some South African mines operate at 25 carats of diamond per 100 cubic meters of rock or about 2 grams of diamonds per 100 tons of rock. Macles diamonds from the Argyle pipe, Western Australia.