A 507½-carat gem discovered in South Africa has become most expensive rough diamond ever sold.Hong Kong’s Chow Tai Fook Jewellery Company bought the Cullinan Heritage stone for $35.3m.Petra recovered the gem, the 19th largest ever found – described as the size of a chicken's egg, from its Cullinan mine near Pretoria in South Africa.

Petra said that record price reflected the "incredible rarity" of stone based on its "remarkable size" and "exceptional colour and clarity".

“It is fitting that the Cullinan Heritage should achieve a sale price of $35.3 million, the highest sale price on record ever achieved for a rough diamond, as it has the potential to produce one of the world’s most important polished gems. The sale proceeds further bolster Petra’s treasury and will be invested in the growth of our core assets.” said Johan Dippenaar, Petra's chief executive.

Chow Tai Fook has yet to outline its plans for the stone.

The world’s biggest diamond, the 3,106-carat Cullinan, was discovered in the same mine in 1905. That was cut to form the Great Star of Africa and Lesser Star of Africa, set in Britain’s crown jewels.

Petra got $9.48 million at a sale in May for a polished 7.03-carat blue gem, also from the Cullinan mine, setting a per-carat record. A carat is a fifth of a gram.

Diamond prices have soared as producers cut output after the credit crunch. Rough diamonds rose 48pc , according to WWW International Diamond Consultants. Demand in China, which overtook Japan as the second-largest gem market, added to the jump in prices.

Petra reported first-half profits of $37.9m (£24.5m) after racking up losses of $88m a year earlier. The shares rose 2p to 60.75p.

Tags: Diamond, Sells, Record, Million, Dollars, South Africa, Jewellery.