What is the world's most expensive diamond?

A blue diamond, one of the rarest gems in the world, was sold for 5.68 million euros at auction at Sotheby's in Hong Kong, becoming the most expensive gemstone per carat in the world.
After an intense bidding, the blue diamond of 6.04 carats reached the amount of more than 940,000 euros (1.32 million) per carat. The price broke the record for 20 years kept the "Hancock Red ', a red diamond which was in its day to pay 660,000 euros ($ 926,000) per carat, Sotheby's said.
The auction house said the buyer was the "Moussaieff Jewellers" in London who have a reputation for acquiring extremely rare gemstones and expensive. The seller was a private collector from Asia.
Blue diamonds have long captivated the rich and powerful and are surrounded by a certain dark mystique.
The famous "Hope Diamond", a gray-blue beauty of 45.52 carats, went through the hands of King Louis XIV of France, Marie Antoinette and American heiress Evalyn Walsh McClean among others. Now rests in the Smithsonian Institution in Washington.
The Sultan of Brunei would have bought another large "blue" that came briefly to the surface in the 1980's.
Although not a large stone, the Sotheby's diamond has an esteemed cut and tone "bright blue ghost" factors that contributed to its record price about 10 times the price per carat of a plain white diamond.
Blue is a result of small amounts of boron in the crystal structure of the stone.
Other colored diamonds with pink and red hues can be mined in many places worldwide, including Brazil, India and Australia, but the "blues" are mostly in the Premier Mine in South Africa.