A flawless vivid blue diamond weighing 7.03 carats sold for a record 10.5 million Swiss francs ($9.49 million), the highest price paid per carat for any gemstone at auction, Sotheby's said.

The rectangular-shaped blue stone, the rarest to enter the international market this year, went to an anonymous buyer bidding by telephone after hectic bidding see-sawed between two callers for 15 minutes.

It was the centrepiece of its semi-annual sale in Geneva, conducted by David Bennett, chairman of Sotheby's jewelry department in Europe and the Middle East, who said the results showed the market's resilience despite the economic downturn.

"This is already a new world record price for a fancy vivid blue diamond and a new world record per carat for any gemstone (at auction)," Bennett told reporters.

"It is fantastic in this market and shows that these rare things are very much in demand," he said.

The final price includes a commission paid by the buyer to the auction house. The stone sets a record price per carat for any gemstone sold at auction of $1,349,752, Sotheby's said.