Diamonds ascend to the Earth's surface in rare molten rock, or magma, that originates at great depths. Carrying diamonds and other samples from Earth's mantle, this magma rises and erupts in small but violent volcanoes. Just beneath such volcanoes is a carrot-shaped "pipe" filled with volcanic rock, mantle fragments, and some embedded diamonds. The rock is called kimberlite after the city of Kimberley, South Africa, where the pipes were first discovered in the 1870s. Another rock that provides diamonds is lamproite.

The volcano that carries diamond to the surface emanates from deep cracks and fissures called dikes. It develops its carrot shape near the surface, when gases separate from the magma, perhaps accompanied by the boiling of ground water, and a violent supersonic eruption follows. The volcanic cone formed above the kimberlite pipe is very small in comparison with volcanoes like Mount St. Helens, but the magma originates at depths at least 3 times as great. These deep roots enable kimberlite to tap the source of diamonds. Magmas are the elevators that bring diamonds to Earth's surface.

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