Sotheby's David Bennett: 'It is exceptionally beautiful and rare'
A rare blue diamond which could set a world record price per carat when it is sold in May has gone on show in London.
Smaller than a penny piece, it is worth between $5.8m (£3.9m) and $8.5m (£5.7m) according to estimates by its sellers.
It weighs 7.03 carats and is one of only a handful of blue diamonds in existence in the world.
The diamond was found in South Africa last year and is on show at Sotheby's, in Mayfair, until Tuesday. It will be auctioned in Geneva on 12 May.
The gem, which was cut from a 26.58 carat rough diamond, was discovered in 2008 at the Cullinan mine.
The mine also produced the 530 carat Great Star of Africa diamond, which is set in the Crown Jewels.
The Gemological Institute of America (GIA) has graded the blue diamond as flawless in clarity, the highest ranking it can give to a diamond.
Cathy Malins, from Petra Diamonds, the company which mined the gem, described it as a unique find.
She said: "At our mine in South Africa we mine between two and three million tonnes of rock a year but we would be very lucky to get maybe one, maybe two, blue diamonds out of that.
"Quite simply we don't know when we'll mine another one".
The scarcity of the gems is in part down to the fact so few places in the world mine for blue diamonds.
The stones get their colour when the chemical boron is present during formation.
Auctioneers say that despite the current economic climate, they have had plenty of interest from prospective buyers across the globe.
David Bennett, Sotheby's Europe and Middle East chairman, said: "It will be bought by somebody who wants the cachet of something that nobody else has - somebody who wants something that is exceptionally beautiful and exceptionally rare.
"I'm optimistic that even in this market the rarity of the stone will carry it along."
In May 2008 a 3.73 carat diamond was sold by Sotheby's at auction for nearly $5m (£3.4m) setting the world record price per carat for any gemstone at auction.
If this diamond sells for the top estimate of $8.5m (£5.7m) it would break that record.
The person who buys it will also get to name the stone.
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