The Blue Moon, a flawless blue 12.03 carat diamond, is expected to smash the record for a diamond at auction.
It's valued at $35m-55m ahead of a sale at Sotheby's New York on November 11.
The record stands at $46.1m, paid for the Graff Pink at Sotheby's Geneva in 2010.
The Blue Moon was discovered in South Africa last year and weighed 29.62 carats in the rough. It's among the largest blue diamonds in the world.
The cutting process, carried out by jewellery conglomerate Cora, took six months.
David Bennett, head of Sotheby's international jewellery division, explained: "The tremendous excitement last year over the discovery of a 29.62-carat blue rough diamond at the Cullinan mine in South Africa has now been proved to have been totally justified.
"Weighing in at 12.03 carats, the 'Blue Moon' diamond is a simply sensational stone of perfect colour and purity, combined with a superb cushion shape.
"Blue, for me, is the most mysterious and magical of all the colours of diamond, and the Blue Moon will now take its place among the most famous gems in the world."
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