In the rough, the diamond was 236 carats when it was extracted from the Jwaneng mine in Botswana, before it was meticulously polished for 21 months, Christie's said in a release issued on Wednesday.

The auctioneer said it was the largest diamond of its type to be offered for sale at auction.

The diamond "has literally been sculpted like a work of art," Rahul Kadakia, who heads Christie's jewellery division in Switzerland and the Americas, said in a statement.

He described the gem as "one of the most beautiful 'white' diamonds Christie's has ever had the honour of offering for sale."

The May 15th auction in Geneva will mark the first time the rock goes on sale, and the buyer will have the privilege of naming it.
   
The American Institute of Gemology has handed the gemstone the top colourless grade "D" and the best clarity grade, "flawless," Christie's said, also hailing the diamond's "absolute symmetry".
   
In this category, the reigning world champion is the Cullinan Diamond, a gem weighing 530.2 carats that is part of the British crown jewels.

Last November, Christie's sold the flawless 76-carat Archduke Joseph Diamond on auction in Geneva for $21.5 million (16.5 million euros).

And in May 2011, a heart-shaped 56-carat diamond was auctioned off for $10.9 million.

The Elizabeth Taylor Diamond, which is in the same D-colour category but weighs in at only 33.19 carats, was meanwhile sold for $8.8 million on auction in New York in December 2011.

Before going on sale in Geneva, the new diamond will go on tour to New York and Hong Kong in April, Christie's said.