Petra Diamonds has discovered a 29.6ct blue diamond at the Cullinan mine in South Africa.
The company said the stone is an outstanding vivid blue with extraordinary saturation, tone and clarity, and has the potential to yield a polished stone of great value and importance.
Cullinan mine is located at the foothills of the Magaliesberg mountain range, 37km north-east of Pretoria in South Africa.
Petra Diamonds acquired the Cullinan mine in 2008 and also operates in Botswana and Tanzania.
Cullinan mine has produced several large stones and frequently yields diamonds larger than 10ct.
The mine has produced more than 750 stones weighing over 100ct, 130 stones weighing more than 200ct and around a quarter of all diamonds that weigh over 400ct.
Petra discovered a similar 26.6ct blue rough diamond in May 2009, which sold for $9.49m at a Sotheby's auction.
The largest rough diamond ever to be discovered came from the Cullinan mine in 1905.
The 3,106ct stone was cut into two diamonds, 530ct First Star of Africa and the 317ct Second Star of Africa, which form part of the British crown jewels.
Other diamonds produced from Cullinan mine include the 353ct Premier Rose, the 426ct (rough) Niarchos, the 599ct De Beers Centenary, the 755ct (rough) Golden Jubilee and the 69ct Taylor-Burton.
Petra Diamonds has interests in six mines, of which five are located in South Africa and one is in Tanzania.
Image: The 29.6ct blue diamond recovered at the Cullinan mine. Photo: courtesy of Petra Diamonds.