From: http://dsc.discovery.com/news/briefs/20050627/diamond.html
By Larry O'Hanlon, Discovery News
June 27, 2005— Diamonds formed in the deepest earth may be the remains of long-dead sea critters and might be evidence of the extreme depths to which Earth can bury its dead, and recycle their remains, say Canadian geologists.
Pressure-revealing minerals within some diamonds from the Jagersfontein mine in South Africa indicate that the carbon gemstones were pressed into existence from graphite at more than 300 miles down.
That's below the crust and into the Earth's deep mantle, far deeper than necessary to make diamonds, said diamond researcher Ralf Tappert of the University of Alberta in Edmonton.