“The stone is a D colour Type II diamond of exceptional size and clarity, and is a magnificent example of the large, high quality diamonds for which the mine is known,” it said in a statement.
“The company will update the market when finalised on the expected timing for the sale of this stone, which is expected to be in the second quarter of this financial year.”
Type II diamonds have no measurable nitrogen impurities, meaning they are often of top quality in terms of colour and clarity.
The mine is renowned for the discovery of the Cullinan diamond in 1905, which was the largest rough gem diamond ever found.
The two stones cut from it, the Great Star of Africa and the Second Star of Africa, are part of the Crown Jewels of the UK.
In June, the company found a rare, 122.52 carat blue diamond at the mine.
Last year, Petra sold a 25.5 carat blue diamond for US16.9 million (about R181.9m).