The rare and flawless "Blue Moon Diamond" sold for $48.4 million to a Hong Kong buyer on Wednesday, setting a world record for a gemstone at auction, Sotheby's said.

The cushion-shaped diamond, mounted on a ring, has the top grading of fancy vivid blue and weighs 12.03 carats. It had a pre-sale estimate of $35 million to $55 million.

"It is a new record price for any gemstone and per carat," David Bennett, worldwide chairman of Sotheby's international jewelry division, told a packed showroom in Geneva that erupted into applause.