I should be denotative here and say that a kimberlite the main host rock for diamonds, is classified as an an ultra-mafic rock. It is defined as a tuffaceous peridotite, or a porphyritic peridotite breccia, or tuffisite. (a tuff is a volcanic ash rock) One of the expressions of a kimberlite is as diatreme. The definition of a diatreme is an explosive volcanoe that pierces to surface or subcrops into a sedimentary rock. In fact kimberlites and other diatremes of differing chemistry may penetrate just about any host rock. A peridotite is a rock made up mainly of olivine and lesser ferromagnesian minerals. Olivine is a silicate of either iron or magnesium. These rocks are believed to originate in the upper mantle, below a diamond, pyroxene, chrome oxide and pyrope garnet forming layer called the chromite-harzburgite layer.
We now know that five types ofrock may give rise to diamonds. kimberlite, orangeite (a near relative of kimberlite), lamproite, lamprophyre, and alpine peridotite. The latter has rare diamonds. Another type of rock called a minette, is theorized to contain diamonds but this is so far unproved. Alpine peridotite and lamproite do not form diatremes. Lamproite is not ultra-mafic but medium to felsic and is not nearly as erodeable as kimberlite. Lamproite makes up 60% ofthe rock mass in mesa and buttes in the Colorado Wyoming district. Although diamond is rare in lamproite, the world's richest mine by caratage was found in that type of rock in Australia. Also a mine in Arkanasa called the Crater of Diamonds was operated briefly by Debeers in 1948 and is in a lamproite host rock. It is now a state park. Large diamonds have been found in the Missouri Arkansas area of up to 30 carats. A diamond mine operated briefly in Kelsey Lake Colorado under Redauram Resources in the 1990's. Kimberlites and rocks known to usually bear diamonds are found in many states of the lower 48. Kentucky, Missouri. NY State, Michigan, the Dakotas, Florida, Arkansas, Colorado Tennessee, Kansas, Wyoming, and Montana. Kimberlites and other rocks with a fair amount ofdiamonds in situ are found in NY State, Colorado, Arkansas, Wyoming and Michigan.
The area where the most diamonds have been found in drift-soil in the world is in a former gold panning area of Michigan. Over 20,000 diamonds were recovered mostly in sluice boxes in the 19th century. The largest was a perfect green gem of 70 carats. The source has never been found of course, but it is thought to be north of there in Canada. I have technical arguments against this theory based on the average transport distance of drift float.
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