A flawless blue diamond that fetched $7.98m (£3.93m) has set a record as the most expensive gemstone per carat bought at auction.
Moussaieff Jewellers of London bought the stone from a private Asian collector at a Sotheby's auction in Hong Kong.
Bidding for the emerald-cut 6.04 carat diamond lasted eight minutes.
The stone sold for $1.32m per carat, easily beating the previous record of $926,000 per carat.
That was set by a red diamond sold in New York 20 years ago.
"The bidding was fast and furious and you know it's not as if one could go out and buy another one," said Quek Chin Yeow, head of Jewellery Asia at Sotheby's.
"It's as rare as getting a Renoir or a wonderful Rothko."
"When you look at the stone, it sings," he said. "The cutting is wonderful."
There is increasing demand for coloured diamonds among jewellery collectors in Europe, the United States and Asia.
Blue diamonds are mostly found at the Premier Mine in South Africa.
The Hong Kong sale on Monday raised a total of $40.8m.
An emerald owned by the American songwriter and lyricist Irving Berlin sold for $2.64m.