DIAMONDS — Like a shot-gun blast from below

Diamond deposits begin forming 150-200 km below the thickest portion of the earth’s crust, where the right combination of cool temperatures and high pressures converts carbon not into the more common graphite but into diamonds. At this depth, the precious stones crystallize and are preserved in dull-green rocks called peridotite or ecologite.

The diamonds may rest there for hundreds of millions years, until gas-charged magma circulating at even greater depths finds a crack in the earth’s crust and begins a rapid ascent at speeds of 10-30 km per hour.

Passing through the diamond window, the magma incorporates fragments of diamond-bearing peridotite and ecologite. These fragments continue to break into smaller pieces during the 5-to-15-hour climb to the surface, releasing diamond crystals into the magma.

Within 3 km of the surface, pressures drop and the gases in the magma begin to expand. Unable to contain the gases in solution any longer, the magma literally explodes into the top portion of the crust in a shower of gas and rock fragments.

The resulting kimberlite pipe resembles an ice-cream cone, has a surface area of about 5-30 ha and consists of three distinct zones that stretch to a depth of about 2,300 metres: the crater zone, the diatreme zone and the thin root zone.

The diatreme zone, which represents most of the pipe, contains the greatest number of diamonds and the most consistent grade. The crater zone is often non-existent, having been eroded away by millions of years of weathering.

There have been three to five episodes of pipe formation over the past several billion years, but economic kimberlites are confined to a range of 52 million years old to just over a billion years old. Unlike conventional volcanoes, there do not appear to be any kimberlite volcanoes active today.