Brussels, Belgium ? The fifth ‘C’: colossal.

The world’s 15th-largest rough diamond was sold Monday in Antwerp for $12.4 million. South African Diamond Corp. purchased the golf ball-sized gem, dubbed the “Lesotho Promise” after the tiny, impoverished Southern African country where it was found.

The diamond has 603 carats and has been graded “class D,” which is the clearest quality with no color blemishes. South African Diamond Corp. said it plans to turn the raw stone into one cut gem above 60 carats in addition to other, smaller stones, with a final sale price of more than $20 million.

The largest diamond ever found, the Cullinan, was the size of a bowling ball at 3,106 carats in the rough.

That finished stone is set in Britain’s Imperial Sceptre as part of the Crown Jewels.