The largest gem-quality rough diamond ever found was called The Cullinan. It weighed an astonishing 3,106.75 carats. From this stone, nine stones were cut; the largest was The Cullinan I which weighs 530.4 carats. It resides in the Royal Sceptre of the British Crown Jewels. Judging of famous diamonds sold at auction in recent times, the Cullinan I is likely worth over $400 million USD. This is definitely where fairytales come from.
International Diamond Jewelers 