The company said the stone, which has potential to yield a polished stone of great value and importance, is an outstanding vivid blue with extraordinary saturation, tone and clarity.
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 Diamonds has interests in six mines, of which five are located in South Africa and one is in Tanzania.
