Describing the egg-shaped gem as the "greatest" of its kind to ever appear at auction, the British firm said it could fetch USD 28 to USD 35 million in the auction, where another rare yet smaller diamond will also be sold.
"It's like [a] small chicken egg," a Hong Kong-based Sotheby's spokeswoman told AFP when she described its size.
Quek Chin Yeow, deputy chairman of Sotheby's Asia, said at a preview yesterday in London that the D colour flawless gem would be the "greatest white diamond to be sold at auction".
"The white diamond ... Is a new stone. The rough was mined in 2011," Quek said.
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It was discovered in a deep mine in an undisclosed southern African country, where it weighed 299 carat as a rough stone.
A "Premier Blue Diamond" at 7.59-carat is also set to be auctioned off in October and could fetch up to USD 19 million.
Hong Kong has become a centre for jewellery auctions thanks to growing wealth in China and other parts of the region, as well as the region's increasing appetite for art.