Beneath the thickest portions of every craton, where pressures are very high, but temperatures are relatively low (considering the depth) due to the overlying, cool crustal rocks, carbon is found stable as diamond.
Kimberlites pass through the diamond stability field on their ascent, picking up pieces of the mantle containing the diamonds and transporting them to the surface.
Many kimberlites do not pass through the diamond stability field and therefore do not have a diamond content.
In the case below the central kimberlite has sampled the diamond stability field whereas the pipe on the right has not.
