A South African company bought the world's 15th-largest rough diamond Monday, paying more than $12 million for the golf-ball sized gem found in the tiny Southern Africa country of Lesotho, officials said.

South African Diamond Corp., the overseas-based manufacturing arm of famed British gem seller Graff Jewelers, purchased the 603-carat stone for $12.4 million, said Lesotho's Natural Resources Minister Mamphono Khaketla, whose government co-owned the gem with a private concern, Gem Diamonds.

"We called the diamond the 'Lesotho Promise.' We promise you more of the same and better in the future," Khaketla told reporters in the Belgian city's diamond district.

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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. Experts have graded the stone "class D," saying it is of the clearest quality, with no color blemishes.

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.

The Lesotho Promise was found in August at the Letsing mine, where in 1967 a 601-carat stone was discovered and was named the Lesotho Brown because of its color.

The 1.8 million people of Lesotho, a tiny mountain kingdom completely encircled by South Africa, are among the world's poorest.