Offers from prospective buyers are to be submitted from 18 August and the process will close on 12 September 2014, the company said.
In 2008, Petra Diamonds bought a 37% stake in the Cullinan mine from De Beers and increased its interest to 74% in late 2009 with the acquisition of Al Rajhi’s holding in the mine.
The company continues using block cave mining methods to mine the underground resource.
Since 2008, the company has recovered various diamonds, including a 39ct blue diamond sold for $8.8m, a 26ct blue diamond sold for $9.4m in 2009, a 507ct white diamond sold in 2010 for $35.3m and a 25.5ct blue diamond sold for $16.9m in 2013.
