Miners in South Africa have found a white diamond about the size of a chicken egg. The diamond may be among the world's top 20 high-quality gems. It was discovered at the Cullinan mine, northeast of Pretoria.

Petra Diamonds Limited chief executive says the 507.55-carat gem was of exceptional color and clarity. No official value has been given for the diamond, but the company’s Finance Director is quoted as saying it could be worth 20-million dollars. It weighs just over 100 grams.

The mine also produced the largest diamond ever found, the Cullinan, at 3,106 carats in the rough. That rough stone was cut into 9 large and 100 small finished stones. The largest of these is set in Britain’s Royal Scepter, which is part of the Crown Jewels.