Mining group Petra Diamonds has discovered a 507 carat stone, which could be worth over $20m (£12.5m), the group announced yesterday. It was found with three other large diamonds at the famous Cullinan mine. Last year, a 480 carat diamond made the firm $18m. Yesterday’s discovery is one of the 20 biggest diamond finds in the world. Diamonds found in the Cullinan mine have been used in the British Crown Jewels.