The stone- which weighs just over 3.53 ounces - was estimated to be among the world's top 20 high-quality rough diamonds. It was discovered in September at South Africa's Cullinan mine.
London-listed diamond supplier Petra Diamonds Ltd. said the gem was purchased by Hong Kong-based private jewelry retailer Chow Tai Fook Jewelry Co. Ltd.
The price reflected "the incredible rarity of the diamond, which combines its remarkable size with exceptional color and clarity, and at 507.5 carats it is the 19th largest gem diamond ever discovered," Petra CEO Johan Dippenaar said in a statement.
"It has the potential to produce one of the world's most important polished gems," he added.
The Cullinan mine, located east of Pretoria, South Africa, was sold by De Beers SA to a consortium led by Petra in 2007 for $148 million.
The mine has turned out some of the world's largest gems, including the 3,106-carat Cullinan Diamond that was cut to form the 530-carat Great Star of Africa and the 317-carat Lesser Star of Africa set in the Crown Jewels of Britain.