JOHANNESBURG - Petra Diamonds Ltd. says a diamond the size of a chicken egg has been found at South Africa's Cullinan mine.

The diamond may be among the world's top 20 high-quality gems. It was discovered Thursday at the mine northeast of Pretoria, South Africa.

Johan Dippenaar, the company's chief executive said in a statement Tuesday that the 507.55-carat gem was of "exceptional color and clarity."