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.