Serious Stone: Sotheby’s diamond is said to be as large as an egg.

An 118-carat stone from Africa billed as the world’s “greatest white diamond” has gone on display at a New York auction house.

The oval gem, not yet mounted after it was mined and cut two years ago, is being showcased at Sotheby’s Manhattan headquarters and has a pre-sale estimate of between $28m (£18m) and $35m (£22m).

In terms of size, quality, polish and colour, “this is the greatest white diamond we’ve ever had the privilege to sell,” said Quek Chin Yeow, deputy chairman of Sotheby’s Asia and an international diamond expert.

The stone was discovered in 2011 as a 299-carat rough diamond in a southern African country whose name Sotheby’s has declined to disclose.

Auction house officials said the owner wishes to remain anonymous.

The current record for any white diamond is $26.7m (£17.1m). That pear-shaped stone was over 101 carats and was sold at Christie’s in Geneva last spring.

Three other white diamonds over 100 carats have been auctioned by Sotheby’s in 1990, 1993 and 1995.

Carefully and meticulously worked over a period of months, the resulting 118-carat stone is one of a number of jewels to be auctioned in Hong Kong on October 7.

Others include a flawless, round, 7.6-carat blue diamond, worth an estimated $19m (£12m).

It is the largest, most significant such diamond graded by the Gemological Institute of America.

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