Blue diamond worth $25.6 million hints at Earth's origins

At the New York headquarters of Cora International, Suzette Gomes is spellbound by a sight of rare beauty. "You can't describe that blue," said Gomes, chief executive officer of the diamondcutting company. "You drown in it." "That blue" refers to the Blue Moon diamond.

Cora paid $25.6 million for the uncut, 29.6-carat stone in February. Coloured diamonds are the world's most expensive stones. A 14.82-carat orange diamond sold for $36 million at Christie's International in Geneva in November, setting a record $2.4 million a carat.

The same month, Sotheby's sold the Pink Dream, a 59.6-carat pink stone, for $83 million. Coloured stones have become popular in the past decade, according to Alan Bronstein, vice president of the New York-based Natural Colour Diamond Association. He's been buying, selling and analysing the stones for 35 years. There's no price index for them because they're rare.

The Blue Moon may provide clues about the forces at play deep within the Earth when the diamond was created at least a billion years ago. The blue is the result of boron traces that lodged inside blue diamonds as they formed about 100 miles (161 kilometres) underground.

Jeffrey Post, curator of the National Gem and Mineral Col lection at the Smithsonian Institution in Washington, studies coloured diamonds for clues about where they come from. He's examined about 25 to 30 blue diamonds, including the famed 45.52-carat Hope Diamond.

Most offer few clues because they weigh less than a half-carat and their origins are unclear. "This will be one of the very few diamonds that are actually sizable, and that we know where it comes from," he said.