Illustration of the tectonic setting for the formation of diamonds in California. Based on calculated thermal profiles, a window into the diamond stability field opens in a subducting slab of oceanic crust around 80 to 110 km in depth. Carrier magmas, such as lamproite or basanite transport diamond-bearing eclogitic material to the surface where it erupts as a maar volcano. Diamonds are postulated to be preserved in the maar crater facies and in the underlying diatreme pipe in a setting analogous to a kimberlite pipe.