The 507-carat stone that could have a value of over $ 20m (£ 12.5m) was found with three other large diamonds.
The mine also produced the largest diamond found, the Cullinan at 3,106 carats in the rough. At the finished stone is set in Britain's Imperial Scepter as part of the crown jewels.
The world's largest certified diamonds are the 3106-carat Cullinan, was found at the mine near Pretoria, South Africa in 1905. It was cut to form the Great Star of Africa and the Lesser Star of Africa set in the crown jewels in the United Kingdom.
Petra sold a 7.03 carat blue diamond, cut from a 26.58-carat rough stone found in the mine for 9.48 million U.S. dollar in May.