A 118.28-carat white diamond, set to break records, is to go under the hammer in Hong Kong in October, auction house Sotheby's said Tuesday.

Describing the egg-shaped gem as the "greatest" of its kind to ever appear at auction, the British firm said it could fetch $28 to $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 Monday preview in London that the D colour flawless gem would be the "greatest white diamond to be sold at auction".

The firm said the sale could beat previous records at a diamond auction last year, when a 101.73-carat diamond was sold at $26.7 million.

"The white diamond ... is a new stone. The rough was mined in 2011," Quek said.

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 $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.