It said the diamond was recovered this month and it is expected to be sold during the company's next tender in February.
Petra said the 299 carat stone was the third-largest high-quality white diamond recovered at Cullinan mine since Petra acquired its stake in 2008.
The other largest high-quality stones were the 507-carat Cullinan Heritage and the 424-carat Legacy of the Cullinan Diamond Mine.
Mathew Nyaungwa, Editor in Chief of the African Bureau, Rough&Polished