The 603-carat (120 gram) diamond, named after the tiny African mountain kingdom where it was found, went under the hammer at the Antwerp Diamond Centre and was sold to the South African Diamond Corporation, owner of luxury jewellers Graff.
The 10th largest white diamond ever to be found, it will be cut into a large heart-shaped diamond and several smaller stones which will then be sold.
The uncut diamond is a third bigger than a golf ball.
Johnny Kneller from the South African Diamond Corporation said that once it was cut up, he expected to sell the stones for a third more than the auction price.
"We can't say for sure but we hope it's going to fetch over US$20 million,” he said.