One of the rarest gems in the world, a flawless blue diamond, has sold for 7.98 million U.S. dollars at a Sotheby's auction in Hong Kong, making it the most expensive gemstone in the world, per carat, sold at auction.

After intense bidding, the 6.04 carat, internally flawless blue diamond fetched 61.9 million Hong Kong dollars (1.32 million dollars) per carat. The price smashed a 20-year-old record held by the "Hancock Red" - a red diamond, which fetched 926,000 dollars per carat at the time, Sotheby's said.

Sotheby's said the buyer was "Moussaieff Jewelers" in London which has a reputation for acquiring extremely rare and costly gemstones. The seller was a private Asian collector.

Blue diamonds have long captivated the rich and powerful - shimmering with a certain dark mystique. The famous "Hope Diamond," a 45.52 carat gray-blue beauty, was passed down through the ages by King Louis XIV of France, Marie Antoinette and American heiress Evalyn Walsh McClean among others. It now rests in the Smithsonian Institute in Washington DC.

The Sultan of Brunei reportedly bought another massive "blue" which surfaced briefly in the 1980s.

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 - 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.