LONDON (AFP) - An extremely rare blue diamond sold for £6.2 million (S$11.8 million) at a London auction on Wednesday, setting a new world record for price per carat.

The fancy deep-blue diamond weighing 5.30 carats sold at Bonhams Fine Jewellery sale in central London for a record US$1.8 million (S$2.3 million) per carat, beating the previous record of US$1.68 million.

The gem, which was bought by international diamond house Graff Diamonds, attracted bids from within the auction room and around the world via 25 telephone lines and smashed pre-auction estimates of £1.5 million.

The diamond is set in a 'Trombino' ring made by Italian jeweller Bulgari around 1965.

"We are delighted with the price it has made," said Mr Jean Ghika, Director of Bonhams Jewellery "It was a sensational stone which charmed everyone who viewed it prior to the sale. Blue diamonds, especially those over 5.00 carats, are extremely rare to see on the market and continue to be highly sought-after. We are honoured to have handled the sale of such a unique gem."

Blue diamonds seldom hit the market and have been coveted by royals and celebrities for centuries.

The most famous example is the 'Hope Diamond', which was bought by King Louis XIV of France in the 17th Century.

The 45.52-carat gem was stolen from the Crown Jewels by thieves during the French Revolution and is now on show at the Smithsonian Natural History Museum in Washington, DC.

The diamond is set within a mount pave-set with brilliant-cut diamonds and courses of baguette-cut diamonds.

The term "fancy" is used to describe a diamond of intense colour. The blue is derived from small amounts of boron impurities within the diamond.