A rare pink diamond auctioned next week could set a new world record if it sells for its estimated price of £17-24 million (20-28 million euros), Sotheby's says.
The 24.78-carat gem, described as "fancy intense pink", would top the previous record set by rival auctioneers Christie's when it sold the blue 35.56-carat Wittelsbach-Graff diamond for £15.2 million (17.8 million euros) in 2008.
"This is undoubtedly one of the most important diamonds to appear at auction in recent years," David Bennett, the head of Sotheby's jewellery division, said in Geneva, Switzerland, where the auction will be held.
Potential buyers had already taken a strong interest, he said, adding: "It's been very, very well viewed."
But diamond experts said Sotheby's price estimate was ambitious.
"The trade price is probably around 750,000 dollars (545,000 euros) a carat," said Martin Rapaport, whose Diamond.Net is a leading source on diamond trading and pricing. That would put the sales price at about £11.7 million (13.7 million euros), not including the auctioneer's commission and sales tax.
"A good price is a million a carat. I think only a consumer is going to pay that," Mr Rapaport said from his office in Israel.
Still, several factors could drive up the price.
First is the rarity and exceptional beauty of the stone, which has remained off the open market since it was sold 60 years ago by New York jeweller Harry Winston.
Unusually for a pink diamond, it has a classic emerald cut with gently rounded corners. Sotheby's says it has a flaw unnoticeable to the naked eye, but may be graded internally flawless after re-polishing.
Economics, too, might play a role.
Global political and economic uncertainty is prompting the super-rich with significant disposable wealth to look for places to keep spare cash safe. In addition to gold, jewels and unique objects of art are considered attractive investments when currencies are sinking.
"There's this idea that there's going to be inflation eventually," said Mr Rapaport. "It's not that the diamonds are necessarily going up, but that the dollar is expected to go down."
There was also the attraction, for some, of being able to easily move 25 million dollars around the world, he said.
"It wouldn't surprise me if we see a lot of people doing a lot of bidding at this auction," Mr Rapaport said.
Sotheby's declined to identify the current owner of the pink diamond, but it is promoting the past owners of several other items on sale at the November 16 auction.
These include a ruby and diamond bangle, necklace and clip earrings once owned by Christina Onassis, the daughter of the Greek shipping tycoon Aristotle Onassis, and an emerald and diamond bracelet belonging to Cristina Ford, the second wife of Henry Ford's grandson.
On Sunday, two days before the jewel auction, Sotheby's is hosting a sale of rare watches and other ornaments, including a mechanical caterpillar from about 1800.
The gold, enamel and pearl device contains a clockwork mechanism that - once wound up - allows it to crawl forward like a real caterpillar. Sotheby's says it was probably made by Swiss watchmaker Henri Maillardet for sale to the Chinese market.
Another item is a small gold and pearl pocket knife with a folding blade - similar to the famous Swiss army knife - which plays music.