A flawless deep-blue diamond was the top lot at a Sotheby's auction in Geneva on Wednesday, selling for 10.27 million Swiss francs ($10.86 million), Reuters reported.

Bought by London luxury jeweler Laurence Graff, who bid by phone, the 10.48-carat stone is about the size of an almond and sold for twice its pre-sale estimate, according to Sotheby's.

The diamond was expected to sell for between 3.2 million to 4.2 million francs, but now holds a world record for highest price of a blue diamond per carat, at $1.04 million per carat.

"It is certainly a world record price per carat for a deep blue diamond," said David Bennett, chairman of Sotheby's international jewelry department, who officiated the auction.

"It is an extraordinary stone, a very, very mystical deep blue."

Bennett also called the diamond β€œthe finest offerings of colored stones in 20 years,” and noted that it came from the Cullinan mine in South Africa, the only mine from which blue diamonds can be found.

According to Sotheby's,β€Ž the stone was "unsullied by secondary colors" and received the highest grade given by the Gemological Institute of America.

Mark Emanuel, of U.S.-based jeweler David Webb, expects that the diamond will be recut.

"It is an extremely rare 10-carat deep blue briolette that can be recut to a 7-carat vivid blue," Emanuel told Reuters.

"Most of these diamonds are bought on the basis of speculating and recutting. That's why we come. As a diamond dealer we look at the potential of a stone.”