Sotheby's showed off a 59.6-carat pink diamond that will be auctioned in the Swiss city of Geneva at a record asking price of $60 million (49 million euros) in November.
"The Pink Star", an internally flawless oval-cut vivid pink diamond, will become the most valuable diamond ever to be offered at auction, Sotheby's said.
"I have had the privilege of examining some of the greatest gemstones in the world over the past 35 years, and I can say, without hesitation, that 'The Pink Star' diamond is of immense importance," David Bennett, chairman of Sotheby's jewellery division in Europe and the Middle East, said in a statement.
"It is difficult to exaggerate the rarity of vivid pink diamonds weighing only five carats, so this 59.60 carat stone is simply off any scale," he said, insisting it belonged in "the ranks of the earth's greatest natural treasures."
The shimmering pink, plum-sized diamond, which was 132.5 carats in the rough, was mined by De Beers in Africa in 1999 and cut and polished over a period of two years by Steinmetz Diamonds, the auction house said.
When it was first unveiled to the public in 2003, it was called the "Steinmetz Pink", but it was renamed after it was first sold four years later.
The 2007 sale price was not disclosed, nor has the identity of its current owner.
The gem, which has received the highest possible colour and clarity rating from the Gemological Institute of America (GIA), is more than twice the size of the 24.78-carat "Graff Pink" diamond that set the world auction record for a diamond and any gemstone or jewel when it was sold at Sotheby's Geneva in 2010 for $46.2 million, the auction house said.