It's a rock for the ages.
A three-billion-year-old diamond the size of a tennis ball - the largest discovered in over a century - could sell for more than $US70 million ($A93.46 million), auctioneer Sotheby's says.
The auction house plans to offer the Lesedi la Rona diamond in London on June 29.
The diamond was unearthed in November in Botswana at a mine owned by Canada's Lucara Diamond Corporation.
It measures 1109 carats, the second-largest gem-quality rough diamond ever discovered. Its name means "our light" in the Tswana language of southern Africa.
The auctioneer said the rough gemstone "of exceptional transparency" could yield the largest top-quality diamond ever cut and polished.
David Bennett, chairman of Sotheby's jewellery division, called the discovery "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 largest diamond ever found was the 3106-carat Cullinan diamond, unearthed in South Africa in 1905. It was cut into nine pieces that form part of the UK Crown Jewels.
The Lesedi la Rona could smash the record price for a diamond of $US48.5 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.
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