An extremely rare blue diamond has become the world's most expensive gemstone per carat bought at auction.

The emerald-cut 6.04 carat stone was bought in Hong Kong by Moussaieff Jewellers of London for a whopping $7.98m, auctioneers Sotheby's said.

The flawless diamond sold for $1.32m per carat, easily surpassing the previous record of $926,000 per carat, which was set by a red diamond 20 years ago.

Mr Quek Chin Yeow, of Sotheby's Hong Kong, said: "The results of today's sale were the latest in a series of triumphs for Sotheby's Hong Kong, which dominates the market at the top end for white and coloured diamonds in Asia."

Bidding for the stone lasted nearly eight minutes, Sotheby's said.

The seller was a private Asian collector, the firm added.