The Pink Legacy, which once belonged to the Oppenheimer family who for decades ran the De Beers diamond-mining company, was snapped up by American luxury brand Harry Winston, part of the Swiss Swatch group.

“$2.6 million per carat. That is a world record per carat for a pink diamond,” said Francois Curiel, head of Christie's in Europe, of the price that included all fees and commissions.

“This stone is for me the Leonardo da Vinci of diamonds.”

The Pink Legacy's classic rectangular cut is traditionally used for white stones, but is rare for pink diamonds. Photo: AFP

The 18.96-carat diamond was discovered in a South African mine around a century ago, Christie's said. It was probably cut in the 1920s and has not been altered since.

The stone was immediately rechristened the “Winston Pink Legacy” by its buyers.

Christie's international head of jewellery, Rahul Kadakia, described it as “one of the world's greatest diamonds”.

The rectangular-cut stone has been graded “fancy vivid” — the highest possible grade of colour intensity — as it has no trace of another hue like purple, orange or brown.

Most pink diamonds weigh less than one carat and those in the top colour category with more than 10 carats are virtually unheard of at auction houses.

The Pink Legacy is classed as Type IIa, meaning it is extremely chemically pure, a category only two percent of diamonds fall into.

“These are stones that have little if any trace of nitrogen,” said Kadakia, adding that this often gives Type IIa diamonds “exceptional transparency and brilliance”.

Emerald cut

“Imagine a domino that you have cut the corners off of,” Jean-Marc Lunel, an international jewellery specialist at Christie's, recently told AFP, describing the stone's unusual shape.

He pointed out that the diamond has a “classical so-called emerald cut” — standing out from the typical cuts used today, which are more rounded and multi-faceted.

The classic rectangular cut is traditionally used for white stones, but is rare for pink diamonds.

Christie's said the Pink Legacy is “the largest and finest Fancy Vivid Pink diamond ever offered at auction by the company”, calling the stone “incomparable”.

“It is probably the most beautiful (specimen) ever presented at public auction,” Lunel said.

Christie's said only four vivid pink diamonds of over 10 carats have ever been offered at auction.

One of them, the nearly 15-carat Pink Promise, was sold last November at a Christie's auction in Hong Kong for $32.5 million. 

That amounts to $2.176 million per carat, which was previously the world auction record price per carat for any pink diamond.

The Pink Legacy's record-smashing price accounted for nearly half the takings at Tuesday's auction, which saw total sales of more than $110 million, according to Christie's spokeswoman Alexandra Kindermann.

While 86 percent of the lots found a buyer, a big heart-shaped white diamond of 53 carats, expected to fetch around $3 to 5 million, went unsold.

Another Geneva auction is set cause a buzz on Wednesday when Marie Antoinette's diamonds and pearls go under the hammer at Sotheby's.

The dazzling treasures, which have not been seen in public for two centuries, are part of a 100-piece collection held by the Italian royal House of Bourbon-Parma.

A diamond parure composed of 95 diamonds, including five solitaire diamonds that belonged to Marie-Antoinette. Photo: Sotheby's/AFP