That made it the most expensive gemstone in the world, per carat, sold at an auction.

After intense bidding, the 6.04-carat emerald-cut blue diamond fetched US$1.32 million per carat.

The price smashed a 20-year-old record held by the 'Hancock Red' - a red diamond which fetched US$926,000 per carat at the time, Sotheby's said.

According to Sotheby's, the new buyer is Moussaieff Jewellers in London, which has a reputation for acquiring extremely rare and costly gemstones. The seller is a private Asian collector.

Sotheby's sale of Chinese treasures - looted from Qing palaces by foreign troops - yesterday also fetched record prices, given gritty bidding by buyers from the mainland, Hong Kong and Taiwan looking to repatriate such objects from the West.

A white jade seal which the Qianlong Emperor carved in 1796 to mark his abdication was the most expensive lot, going for HK$46.2 million (S$8.8 million).

It broke the record for any Chinese white jade or imperial seal ever sold at an auction.

REUTERS, BLOOMBERG