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April 28 (UPI) -- A 15-carat blue diamond found in South Africa last year -- the largest diamond of that color ever put up for auction -- has sold for close to $60 million, blowing past even the top-end estimates for the gem. Sotheby's in Hong Kong said that the De Beers Cullinan Blue diamond sold to an anonymous buyer over the telephone on Wednesday for $57.47 million.
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The sale featured a bidding war between two buyers that lasted for eight minutes before the final hammer came down.
Before the auction, Sotheby's estimated that the step-cut gem could fetch as much as $48 million. The final price was roughly $10 million higher.
The Cullinan Blue gemstone is the largest internally flawless vivid blue diamond that the Gemological Institute of America has ever graded, the auctioneer said. The GIA says it's given its top color grading -- "fancy vivid blue" -- to no more than 1% of blue diamonds it evaluates.
The stone was cut from an exceptional rough stone mined in South Africa a year ago.
Sotheby's says that blue diamonds with the size and quality of the De Beers diamond are "exceptionally rare." Only five other diamonds larger than 10 carats ever have come to auction and none exceeded 15 carats. The De Beers stone is 15.10 carats.
The De Beers stone barely missed setting a sales record for blue diamonds. The record-holder is the 14.62-carat Oppenheimer Blue diamond, which sold for $57.5 million six years ago.
Blue diamonds are among the rarest of all colored diamonds. The blue hue is normally caused by small amounts of boron within the crystal structure. This differs entirely from pink diamonds, which get their color from distortions within the lattice structure. Diamonds can exhibit a number of colors -- including orange, yellow, green and brown -- and most get their color from small amounts of chemicals inside.
