A huge, internally flawless diamond from India's fabled Golconda mines was sold at auction in Geneva on Tuesday night for a record 20.35m Swiss francs (£13.5m), Christie's said.
The rare, colourless stone -- weighing 76 carats and roughly the size of a large strawberry -- once belonged to Archduke Joseph August of Austria (1872-1962), a prince of the Hungarian line of the Habsburgs.
It fetched more than double the price paid for it at auction almost two decades ago.
"It is a world record price per carat for a colourless diamond," François Curiel, director of the international jewellery department at Christie's, said.
"The market is not on the best form at the moment. The sale tonight was almost flabbergasting."
He added that the buyer wished to remain anonymous, but revealed that the underbidder was Fred Mouawad, an international dealer with offices in Dubai and Geneva.
The diamond was sold by American jeweller Black, Starr & Frost.
"My understanding is that this stone is going to a museum and it will probably be the centrepiece," said Alfredo Molina, chairman of Black, Starr & Frost.
The diamond was the star lot at Christie's semi-annual jewellery sale in Geneva. The sale as a whole fetched £50m, with 290 of 348 lots sold.
Historical diamonds originating in the Golconda mines, virtually exhausted by the 18th century, include the Koh-i-noor, now in the British crown jewels, and the blue Hope Diamond, part of the Smithsonian Institute in Washington DC, Christie's said.
"The Archduke Joseph Diamond is the finest and largest perfect Golconda diamond ever to appear at auction," Rahul Kadakia of Christie's said.
"It is comparable in its noble lineage and superb quality to the legendary Koh-i-noor."
Records show the Archduke deposited the stone in the vault of the Hungarian General Credit Bank in 1933.
"Three years later it was sold to a European banker, and kept in France, locked away in a safe deposit box, where fortunately it remained undiscovered during the second world war," the auction house said.
It surfaced at auction in 1961 and again at Christie's in November 1993, netting £4m at the time. The stone was subsequently "slightly recut".