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Why It's So Hard to Put a Price on the World's Biggest Diamonds

  • Stones so rare only small group of super-rich can afford them
  • To gauge value, miner Gem Diamonds cuts some of its own rocks
A pile of colorless and colored uncut diamonds sit on a sorting table before processing at DTC Botswana, a unit of De Beers, in Gaborone, Botswana, on Thursday, Oct. 25, 2012. XXX SECOND SENTENCE HERE XXX.
Photographer: Chris Ratcliffe/Bloomberg
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When miners unearth the world’s biggest and rarest of diamonds -- like the golf-ball-sized, 357-carat rock found this year in the southern African kingdom of Lesotho -- figuring out what they are worth can prove almost as difficult.

Gem Diamonds Ltd., which specializes in digging up stones that only a few billionaires are likely to buy, has been taking some unusual steps to confront that dilemma. The London-based mine owner is replicating on a small scale what middlemen normally do. It cuts, polishes and re-sells some diamonds to get a better sense of what the market is for the world’s biggest ones. In September, the company had its biggest sale ever when its prized discovery from Lesotho fetched $19.3 million.