The Rarest Gems in the World : Blue Diamonds

Blue diamonds have captivated the rich and powerful shimmering brilliance with a certain mystique. The famous Hope Diamond, a 45.52 carat grey-blue in beauty, was passed down through the ages from King Louis XIV of France, Marie Antoinette and American heiress Evalyn Walsh McClean among others. It can be found today in the Smithsonian Institute in Washington, DC.

One of the rarest gems in the world, a flawless blue diamond, has sold for $US7.98 million ($A8.9 million) at a Sotheby's auction in Hong Kong, making it the most expensive gemstone per carat in the world sold at auction. The Sultan of Brunei reportedly bought another massive "blue" which surfaced briefly in the 1980s.

After intense bidding, the 6.04 carat, internally flawless blue diamond fetched $HK61.9 million, or $US1.32 million ($A1.47 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, auction house Sotheby's said. Sotheby's said 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.

While not a large stone, the Sotheby's diamond has an esteemed cut and "fancy vivid blue" hue - factors which contributed to its blockbuster price of roughly 10 times the per-carat price of regular white diamonds. The blue hue is a result of trace amounts of boron in the stone's crystal structure. Other coloured diamonds with pink and red hues can be mined in multiple locations across the globe including Brazil, India and Australia, but "blues" are mostly found at just one site on earth - the Premier mine in South Africa.