A massive diamond with a possible price tag of more than �36m has been discovered at a mine in South Africa.

The 29.6 carat blue diamond, described as being "exceptional", was dug up at the Cullinan mine near Pretoria - owned by Petra Diamonds.

Chief executive Johan Dippenaar said: "The stones in the last year or so are selling well above $2m (1.2m pounds) per carat. That's not my quote, that's updates in the market."

However, analyst Cailey Barker at brokers Numis said it could expect to fetch less - between $15m (9m pounds) and $20m (12m pounds) - at auction.

The mine, owned by the firm since 2008, was also where the Cullinan Diamond was found in 1905 - described as the largest rough gem diamond ever recovered and weighing 3,106 carats.

Other notable diamonds found in the mine include a 25.5 carat Cullinan blue diamond, found in 2013 and sold for $16.9m (10m pounds), and a diamond found in 2008, known as the Star of Josephine, which was sold for $9.49m (5.7m pounds).