A diamond the size of a tennis ball which is the largest discovered in more than a century could sell for as much as £50m, according to Sotheby's.
The auctioneers will offer the Lesedi la Rona diamond for sale in London on June 29.
The gemstone was unearthed in November in Botswana at a mine owned by Canada's Lucara Diamond Corporation.
It measured 1,109 carats, making it the second-largest gem-quality rough diamond ever discovered.
Its name means "our light" in the Tswana language.
The auctioneer said the rough gemstone could yield the largest top-quality diamond ever cut and polished.
The largest diamond ever found was the 3,106-carat Cullinan diamond, unearthed in South Africa in 1905. It was cut into nine pieces that form part of the UK Crown Jewels.
David Bennett, chairman of Sotheby's jewellery division, called the discovery of the three billion-year-old diamond "the find of a lifetime" and the auction unprecedented.
"Not only is the rough superlative in size and quality, but no rough even remotely of this scale has ever been offered before at public auction," he said.
The Lesedi la Rona could smash the record price for a diamond of $48.5m (£33.5 million), paid at a Geneva sale last year for the 12.03-carat polished Blue Moon diamond.
Hong Kong billionaire Joseph Lau picked it up as a gift for his seven-year-old daughter.