Petra discover a 138.57-carat white diamond.
The diamond is classified as Type IIa, D colour, meaning it has high clarity, and will be offered for sale as part of the company’s upcoming sales process in Johannesburg next month.
Earlier this year the company recovered a 121.26-carat white diamond from the same mine, which it sold for $6m (£4.6m).
The Cullinan mine is known for its large and high-quality stones. The largest rough diamond ever found was recovered at the mine in 1905 – the 3,106-carat diamond was cut into the two most important diamonds which form part of the Crown Jewels in the Tower of London.