Blues of consequence are among the rarest of the rare among fancy colour diamonds. Since the Cullinan mine in South Africa was acquired by Petra from De Beers in 2008, it has produced six important blue diamonds. The most recent was the stunning 122.52-carat blue, recovered in June 2014, that sold this September for $27.6 million, or $225,269 per carat. In February 2014, a stunning blue of incredible intensity, weighing 29.6 carats, also from the Cullinan mine, was sold to Cora International for $25,555,555, or $862,780 per carat — and that’s for a piece of rough. It remains to be seen if the amount of the investment will be justified by the price tag on the ultimate gem it yields.

In the six years that Petra has worked the Cullinan mine, it has treated somewhere between 16 million and 18 million tons of ore, as well as another 10 million to 12 million tons of tailings. While all that material produced 6 million carats of white diamonds, it turned up just a half dozen blues. Miners don’t mine for blues; they just count themselves quite fortunate that one turns up every so often.

Petra is not the only player in the blue field of dreams. Canadian mining company Lucara Diamond Corp. found a singular 9.46-carat blue at its Karowe mine in Botswana. The rough was sold in July 2014 for $4,515,000, or $477,272 per carat.