The diamond will be sold as part of Petra’s next sales process at the end of the month in Johannesburg.
The Type 2a white diamond is the latest in a trove of monster stones recovered in the last year.
At 1,100 carats it is the largest uncut diamond in the world, Petra’s stone seems like a pebble in comparison.
It was recovered in Botswana last November and its name means “Our Light” in the native Tswana language.
Lucara found three large sized diamonds within the space of 72 hours and experts have attributed the recent trove of gems to a change in the sorting and extraction processes.
Previous processes may have resulted in the loss of a number of larger stones.
In May, Lucara sold another huge stone called The Constellation, found in the same mine. At 813 carats, it fetched an auction price of US$63mln, around US$77,000 per carat, the new record for the sale of a rough gem.
Petra’s Cullinan Mine is renowned for the discovery of the Cullinan Diamond back in 1905, at 3,106 carats it is the largest gem ever discovered. It was cut to form part of the Crown Jewels.
Many of the world’s most famous diamonds have been found at the mine and the group continues to produce high quality gems, as is the most important source of the incredibly rare but highly sought-after blue diamonds.
Recent studies estimate 50-year lifespan remaining at the orebody, with a diamond resource of 195mln carats.
Shares in Petra were up 2% to 120p.