"The source of a diamond is a kimberlite pipe, a form of diatreme--a relatively small hole bored through the crust of the earth by an expanding combination of carbon dioxide and water which rises from within the earth's mantle and moves so fast driving magma to the surface that is breaks into the atmosphere at supersonic speeds. Such events have occurred at random through the history of the earth, and a kimberlite pipe could explode in any number of places next year.
"There is a layer in the mantle, averaging about sixty miles below the earth's surface, through which seismic tremors pass slowly. The softer the rock, the slower the tremor--so it is inferred that the low-velocity zone, as it is called, is close to its melting point. In the otherwise rigid mantle, it is a level of lubricity upon which the plates of the earth can slide, interacting at their borders to produce the effects known as plate tectonics. The so-termed lithospheric plates, in other words, consist of crust and uppermost mantle and can be as much as ninety miles thick. Diamond pipes are believed to originate a good deal deeper than that--and in a manner which, as most geologists would put it, "is not well understood." After drawing fuel from surrounding mantle rock--compressed water from mica, in all likelihood, and carbon dioxide from other minerals--the material is thought to work slowly upward into the overlying plate. Slow it may be at the start, but a hundred and twenty miles later is comes out of the ground at Mach 2. The result is a modest crater, like a bullet hole between the eyes."
--Annals of the Former World, John McPhee.