The Lesedi la Rona, the world's second largest rough diamond, is expected to realise up to £48m ($70m) at Sotheby's London on June 29.
That would make it the most valuable diamond ever sold at auction.
Lesedi la Rona means 'Our Light' in Tswana
The record stands at $48.4m, set for the Blue Moon of Josephine at Sotheby's last year.
The Lesedi la Rona was discovered in a mine in Botswana last year and weighs around 1,109 carats.
It's second only to the Cullinan diamond, found in 1905, which weighed 3,016 carats in the rough.
The Cullinan produced the world's largest cut diamond, the 530 carat Great Star of Africa that forms part of the British crown jewels.
The Lesedi la Rona, when cut, has the potential to produce an even larger stone.
David Bennett, worldwide chairman of Sotheby's Jewellery Division, said: "The Lesedi la Rona is simply outstanding and its discovery is the find of a lifetime.
"It is a huge honour for Sotheby's to have been entrusted with its sale. Every aspect of this auction is 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."
The diamond's name was chosen in a public competition and means "Our Light" in Tswana, the official language of Botswana.
Please sign up to our free newsletter to receive exciting news about watches and jewellery auctions.