The Cullinan diamond mine

Where the largest rough diamond ever found came out of the wall

The Cullinan mine sits in the town of the same name, 40 km east of Pretoria in Gauteng. It works a carrot-shaped volcanic pipe, a plug of diamond-bearing rock that covers 32 hectares at the surface and narrows downward, and it opened in 1902 as the Premier Mine of Thomas Cullinan.

On 25 January 1905 the surface manager, Frederick Wells, found a crystal of 3,106.75 carats, more than 621 grams, the largest rough gem-quality diamond ever found. It was named for the mine's owner and presented to King Edward VII, and among the stones cut from it the 530-carat First Star of Africa and the 317-carat Second Star of Africa are set in the British crown jewels.

That was only the start. Cullinan has produced more than 750 stones of over 100 carats and more than a quarter of all the world's diamonds above 400 carats, among them the 755-carat rough of the Golden Jubilee, the 426-carat Niarchos, the 353-carat Premier Rose and the 275-carat De Beers Centenary. The Taylor-Burton diamond, 69 carats polished, came from here too. In 2009 a 507-carat stone turned up, and Petra Diamonds sold it the next year for $35.3 million, then a record for any rough diamond.

The mine is also the only significant source of blue diamonds in the world. A 26.6-carat blue rough found in 2009 sold as a cut stone for just under $10 million at Sotheby's, another deep blue from Cullinan set a world record for value per carat, and the company announced a 25.5-carat blue in April 2013, a 29.6-carat stone in January 2014 that its chief executive called the most significant blue it had recovered, and a 122.52-carat blue that June.

De Beers sold the mine in 2007 for about R1 billion to a consortium led by Petra Diamonds, which completed the purchase in July 2008. It was renamed the Cullinan Diamond Mine for its centenary in November 2003 and is worked underground.