GENEVA (AFP) - A plum-sized diamond known as the Pink Star goes under the hammer on Wednesday in Geneva in an auction hoped to rake in US$60 million (S$75 million), the highest price ever for a gemstone.
The Sotheby's auction comes a day after rival house Christie's sold an almond-shaped diamond dubbed "The Orange" for US$35.5 million, also a record in its category.
The world's biggest pink diamond, the Pink Star belongs to "the ranks of the Earth's greatest natural treasures", said Sotheby's Mr David Bennett.
"It's really extraordinarily rare," Mr Bennett, the chairman of Sotheby's jewellery division in Europe and the Middle East, told AFP of the gem glimmering inside a well-guarded showcase.
"Very, very few of these stones have ever appeared at auction."
The flawless 59.60-carat pink diamond is the largest in its class ever graded by the Gemological Institute of America (GIA), with the second biggest less than half its size, he said.
The sparkling oval-cut rock, which has received the highest possible GIA colour and clarity rating, will be part of the auction house's Magnificent Jewels auction at a luxury Geneva hotel Wednesday evening.
Set on a ring, it measures 2.69 by 2.06cm, and weighs 11.92g. It would earn Sotheby's around US$5 million per gramme or US$1 million per carat if it fetches its asking price.
Mr Bennett described the estimate as "very reasonable".
"The international demand for exceptional diamonds keeps on growing," he said.
Three years ago, Sotheby's set a record when it sold the Graff Pink diamond for US$46.2 million dollars. Half the size of the Pink Star, it came from a private collection and had not been on the market for 60 years.
"That was an unequalled price for any diamond or jewel ever sold at auction," said Mr Bennett.
"Only a handful of people in the world could consider buying this rock," a gem expert told AFP on condition of anonymity after Sotheby's announced the auction in September.
"Sixty million dollars, that is really a lot of money, and the expenses come on top of that," he said. "To buy this stone, you really have to fall in love - and also think about a possible sale in a couple of years, preferably at a profit."
Analysts note that investors have often turned to diamonds and other jewels in uncertain economic times. They also carry prestige for growing global elites.
The Pink Star was 132.5 carats in the rough when it was mined by De Beers in Africa in 1999, according to Sotheby's, which has not said which country it came from.
It was then cut and polished over a period of two years by Steinmetz Diamonds, and was first unveiled to the public in 2003 under the title of the Steinmetz Pink.
The near-translucent rock was renamed after it was first sold four years later for an undisclosed sum to an unidentified buyer.
Sotheby's declined to name the seller in Wednesday's auction, nor would it say whether the gemstone had been bought and sold again since 2007.
In addition to its colour and clarity ratings, it falls into a rare subgroup of stones with the chemically purest diamond crystals and often extraordinary optical transparency, comprising less than two percent of all gem diamonds, according to the GIA.
"It really ticks all of the boxes in terms of excellence," Mr Bennett said.
Best known for shimmering white translucence, diamonds in fact come in all sorts of colours.
Green diamonds, for instance, are coloured by radioactivity in the ground, blue diamonds get their colour from boron, and yellow diamonds, which in very rare cases turn orange, are coloured by nitrogen.
Once considered a curiosity, coloured diamonds today rake in higher prices per carat than even the most flawless white.
The Orange, a whopping 14.82 carats, fetched the highest price per carat (US$2.4 million) ever paid for a diamond at auction, as well as the highest ever for an orange diamond, Christie's said after Tuesday's sale, also held in Geneva.