A flawless blue diamond weighing 7.03 carats has sold for a record 10.5 million Swiss francs (£6.2 million), the highest price paid per carat for any gemstone at auction, Sotheby’s said.

A flawless blue diamond weighing 7.03 carats has sold for a record 10.5 million Swiss francs (£6.2 million), the highest price paid per carat for any gemstone at auction, Sotheby’s said.

The rectangular-shaped blue stone, mounted on a platinum ring, is the rarest to enter the international market this year.

The anonymous phone bidder, who won after a 15-minute war, has yet to name the gem.

The flawless diamond, which is smaller than a penny, was found in Cullinan mine in South Africa last year, and was the centrepiece of Sotheby’s semi-annual sale in Geneva.

The stone sets a record price per carat for any gemstone sold at auction of $1,349,752 (£883,956), Sotheby’s said.

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