The world's largest flawless blue diamond is set to cross the block at Christie's in Geneva on May 14 with an estimate of $21m-25m.

The origins of the 13.2 carat stone are somewhat mysetrious, although it is thought to have been discovered in South Africa and has been in the hands of the same owner for a number of years.

Blue diamonds are exceptionally rare, particularly the most vivid examples. The size of the present example is also likely to drive its value higher on the day of the auction.

Rahul Kadakia, Christie's head of jewellery for the Americas & Switzerland, commented: "To have a stone that weighs 13 carats is near impossible. 

"The stone is internally and externally bereft of any imperfections... in simple terms, it's the best blue diamond in the world."

Last year there were a number of major diamond sales. The record for the world's most valuable example was shattered by the 59.6 carat Pink Dream - which hammered for an impressive $83.1m at Christie's Geneva in November.

It eclipsed the $39.3m paid for the pink "Princie" diamond at Christie's New York in April.

The Orange, an 14.8 carat vivid orange stone, was another highlight of the year, selling for $35.5m.  

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