The rare and flawless "Blue Moon Diamond" has sold for 48.6 million Swiss francs ($A68.58 million) to a Hong Kong buyer, setting a world record for a gemstone at auction, Sotheby's says.
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 $US35 million ($A49.60 million) to $US55 million.
"It is a new record price for any gemstone and per carat," David Bennett, worldwide chairman of Sotheby's international jewellery division, said on Wednesday.
The Hong Kong buyer promptly renamed it The Blue Moon of Josephine, Bennett told reporters, noting that it had also set a world record for any jewel at more than $US4 million per carat.
At rival Christie's on Tuesday, a large diamond of a rare pink hue fetched 28.725 million Swiss francs.
Christie's said the stone, named The Sweet Josephine by the Hong Kong-based Chinese client who bought it, set a world record for any pink diamond ever offered at auction.
The same buyer is believed to have bought both star lots at the semi-annual sales in the Swiss city.
The blue diamond stone was found in South Africa's famed Cullinan mine in January 2014. The seller was New York-based jeweller Cora International, according to Sotheby's.