Mining group Petra Diamonds has discovered a 507.55 carat white diamond at South Africa's Cullinan mine, one of the largest high-quality rough diamonds ever found, the firm said on Tuesday.

"This spectacular gemstone was recovered on Thursday 24 September and is currently with experts for analysis," said a statement released by the London-listed company which operates mainly in Africa.

Initial examinations of the diamond which weighs just over 100 grams (3.5 ounces), have shown it to be of exceptional colour and clarity.