London, Apr 26 (IANS/EFE): A rare blue diamond weighing 5.3 carats sold for 6.2 million pounds ($9.4 million) at an auction in London, auction house Bonhams said.

"The previous world record price per carat for a blue diamond was $1.68 mn. This rare gem was bought by Graff Diamonds, the international diamond house headquartered in New Bond Street, London. Bidding came from round the world via 25 telephone lines as well as bidders in the packed saleroom," Bonhams said in a statement.

The piece, which dates to 1965, is made up of a rectangular deep blue diamond mounted on a Trombino ring designed by jeweller Bulgari, one of the favorites of Hollywood stars of the 1960s.

The diamond, which was going at auction for the first time, is one of the rarest gems of its kind, Bonhams said.

"We are delighted with the price it has made. 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 honored to have handled the sale of such a unique gem," Bonhams Jewelry Department director for Europe Jean Ghika said.

The blue diamond brought six times the estimated sale price at Wednesday's auction.

"The extraordinary color blue is derived from small atoms of boron mixing with the carbon atoms in a diamond, affecting the absorption of light passing through the stone and lending it a blue appearance. Blue diamonds are structurally very pure and account for less than 1 percent of all diamonds mined," the auction house said.