A rare blue diamond weighing 7.03 carats is to be auctioned by Sotheby’s.

Photo: Petra Diamonds

The diamond currently belongs to Petra Diamonds, the company that now owns and operates the famous Cullinan mine in South Africa, previously owned by De Beers.  The rough diamond (pictured below with Johan Dippenaar, CEO of Petra) weighed 26.58 carats and was discovered at the Cullinan mine in 2008.

Johan Dippenaar with uncut 26.58 ct blue diamond. Photo: Petra Diamonds

The diamond will be shown in Hong Kong, Paris, New York and London before the sale takes place at the Beau-Rivage Hotel in Geneva on 12th May 2009.  The pre-sale estimate for the diamond is in the range $5.8m to $8.5m, and the buyer will have the additional honour of giving the stone a name.

Exceptional diamonds are often referred to by their per-carat value, and the pre-sale estimate translates into between $825,000 and $1,210,000 per carat.  A 6.03 carat emerald-shaped vivid blue diamond was sold by Sotheby’s in Hong Kong in October 2007 for $7.9 million.  At the time that was a world-record value of $1.32 million per carat.

This exceptionally high per-carat value can be explained by the rarity of a diamond of this colour and purity.  Not only is the diamond blue, which is a rarity in itself, but it is a ‘vivid blue’ and it is also internally flawless.  Colouring in a diamond is usually the result of an impurity at an atomic scale within the crystal lattice of the diamond.  For blue, the impurity is the element boron.

You can see a video of the diamond on the BBC website here.