Diamonds form deep — more than ~150 km down in the mantle, under crushing pressure (recall Unit 1: diamond is pure carbon, but with a different atomic structure from graphite). The puzzle is how they reach the surface without turning back into graphite on the way up.

The kimberlite elevator

The answer is the kimberlite pipe. Kimberlite is a rare, deep-sourced magma that blasts upward extremely fast, dragging diamonds with it and freezing them in before they can change. It erupts in narrow, carrot-shaped pipes (also called diatremes) that punch from the mantle right to the surface. A close relative, lamproite, hosts Australia's famous Argyle mine.

Grade is tiny — a good kimberlite might carry well under a gram of diamond per tonne — but the stones are so valuable that it pays handsomely.

Placer diamonds

Once a pipe erodes, rivers and the sea carry the durable diamonds away and concentrate them as placers, sometimes far from any pipe (recall Unit 2's gravity-sorting). Namibia's coast and seabed host rich marine diamond placers.