A rare flawless blue diamond broke the 20-year per-carat record for a gemstone at an auction on Monday in Hong Kong, Sotheby's auction house said in a release.

The 6.04-carat emerald-cut diamond sold for $7.87 million, or $1.3 million per carat.
(CBC)

The 6.04-carat emerald-cut diamond sold for $7.87 million, or $1.3 million per carat, after eight minutes of intense bidding among seven interested buyers.

The previous record was held by a 0.9 carat red diamond, known as the Hancock Red, sold in New York in 1987 for $912,832 per carat, or a total of $867,580.

Reuters reported the buyer was Moussaieff Jewellers in London, which has a reputation for acquiring extremely rare and costly gemstones. The seller was a private Asian collector.

The vivid blue shade of the diamond is caused by trace amounts of boron in the crystal.

Other coloured diamonds, such as the pink and red hues, can be mined in several countries, including Brazil, India and Australia, but the blue shade is mostly found at South Africa's Premier Mine.