[1] It was found by Frederick Wells, surface manager of the Premier Diamond Mining Company in Cullinan, Gauteng, South Africa, on 25 January 1905.

While internally flawless, the stone has a slight imperfection on the surface that is imperceptible to the human eye, the auction house said.

The mine has produced hundreds of large stones and is famed for its production of blue diamonds.

A similar 26.6-carat (5.32 g) blue rough diamond recovered by Petra in May 2009 was cut into a near perfect stone and fetched just under $10m at Sotheby's.

[6] Another deep-blue diamond from Cullinan was auctioned for $10.8m last year and set a world record for the value per carat.

[10] On 22 November 2007, De Beers, the world's largest diamond producer, announced that it had entered into an agreement to sell the Cullinan mine for R1 billion (US$125 million) to Petra Diamonds Cullinan Consortium (PDCC), a consortium of: On 16 July 2008, Petra announced the completion of the acquisition.