Diamonds are formed in the Earth’s mantle at a depth of 150 to 200 km, and brought to the surface rapidly in a volcanic vent that forms what is called a kimberlite pipe. No one has ever witnessed a kimberlite volcanic eruption, but it is believed that the magma travels at several hundred kilometers per hour as it moves through the Earth’s crust, and may be ejected at supersonic speeds. Kimberlite (named after Kimberley, South Africa, where it was first described) is a type of volcanic rock which comes directly from the Earth’s mantle.
I taught about kimberlites in my high school Earth Science class today, along with other ores formed by igneous processes. I found a good animation of a kimberlite eruption from Diamondex, a Canadian diamond mining company.
We know that the magma must come from a great depth, because diamonds form only at very high pressures. We know that the magma rises extremely rapidly because diamonds are very unstable when there is both a low pressure and high temperature. If the magma were to rise more slowly, the diamonds would recrystallize to graphite, which is what we make pencil lead out of. If this were to happen, the South Africans would mine for pencil lead instead of for diamonds, which of course is not nearly as profitable.
Grace and Peace