A rare blue diamond set a world record in May 2009 when it went under the hammer as part of Geneva's spring auction sales.
The 7.03-carat fancy vivid blue diamond fetched SFr10.5 million ($9.49 million), the highest price paid per carat for any gemstone at auction, Sotheby's said after its Magnificent Jewels sale.
It is the rarest stone to enter the international market this year and had an estimate of SFr6.8-10 million. It went to an anonymous telephone buyer after a 15-minute bidding battle between two callers.
David Bennett, chairman of Sotheby's jewellery department in Europe and the Middle East, said the results showed the market's resilience despite the economic downturn.
The previous record price for a fancy vivid blue diamond was SFr8.7 million for a stone weighing 6.04 carats at a sale in Hong Kong in 2007.
The new owner will have the right to name the stone, which is mounted on a platinum ring.
The blues are the rarest of the diamond family after the reds. There are thought to be only a handful in existence in the world. The blue gemstone owes its distinctive colour to the presence of the chemical element boron during the stone's formation.
"We're talking about an off-the-scale, stratospherically rare stone," Bennett told swissinfo. "It has all the boxes ticked in terms of rarity, quality and perfection. It's an extraordinary stone."
The internally flawless gemstone was cut from a 26.58-carat rough stone unearthed last year at Petra Diamond's Cullinan mine, near Pretoria, in South Africa.
The mine was also the source of the world's largest rough diamond ever found. The 3,106-carat "Cullinan" was discovered in 1905, from which the 530-carat Great Star of Africa diamond originated, which is part of the Crown Jewels at the Tower of London.
"[The mine] has also produced a quarter of the world's diamonds of over 400 carats in size – and it's the world's only reliable source of blue diamonds, which are incredibly rare," said Cathy Malins, spokeswoman for Petra Diamonds.
According to Malins, Petra mines between two and three million tonnes of rock a year, and if it is lucky it gets maybe one or two blue diamonds out of that.
Diamond cutter Gary Monnickendam took seven weeks to produce the final stone from the rough diamond: two weeks of just looking at it, and then five weeks of cutting and polishing.
"As cutters, we live to cut the occasional blue diamond, because it is so exciting. If you get the opportunity to do the best it is like a privilege," Monnickendam told Times Online. "I'm in a state of postnatal depression now; it's like giving birth."
Prices of more mainstream, jewellery-grade diamonds have fallen sharply in recent months due to the recession. But senior jewellery experts are confident that collectors and traders remain hungry for unique items, especially coloured diamonds or historic gems.
"It's a slightly different world market in terms of the economy, but for buyers or collectors of blue diamonds there are so few opportunities to find one, that they have to bid," said Bennett.
In May 2008 a 3.73-carat blue diamond ring sold for SFr9.2 million, setting a world record price per carat for any gemstone at auction.
"The stone I sold last year was half the size of this one and made just over $1.3 million per carat. This one we're estimating at $850,000 per carat at the low estimate. We'll just have to see where it goes," said Bennett before the auction.
"Blue diamonds attract the kind of people who want something nobody else has," he added.
Over the past few years Geneva's reputation has grown as a centre for spectacular jewellery sales, in particular large gemstones.
In November 2007, Sotheby's in Geneva sold a huge, flawless brilliant-cut white diamond, weighing 84.37-carats, for SFr18.2 million ($16.21 million) to Guess Jeans founder Georges Marciano.
The diamond achieved a world record price per carat for a white diamond ($191,980), and was the second-most expensive stone ever sold at auction, only eclipsed by the 100.10-carat "Star of the Season" diamond, which the same Geneva branch sold for $16.5 million in May 1995.
Simon Bradley in Geneva, swissinfo.ch
Geneva has been a major centre for auctions since 1968, especially for jewellery and watches, which it holds twice a year: in May and November.
Jewellery, luxury watches and vintage wine valued at SFr90 million ($79.2 million) are up for auction in the 2009 spring sales.
Antiquorum, which specialises in horology, is auctioning 331 watches from the "collection of a European nobleman", worth SFr5.8 million.
Sotheby's is selling 170 watches and other timepieces on May 10, as well as 350 lots of diamonds and jewellery on Tuesday, worth up to SFr50.2 million. The centrepiece is the 7.03-carat fancy vivid blue diamond.
Phillips de Pury and Company has 173 lots of watches and jewellery on sale worth SFr4.3 million.
Christie's is gearing up for three sales – with watches, fine wine and jewellery on offer. In all it hopes to bring in some SFr36 million.
In May 2008, sales of watches, jewellery and fine wines in Geneva brought in SFr175 million, compared with SFr90 million in November 2008.