'Perfect' Archduke Joseph diamond sells for $21m

One of the world's most admired diamonds has been sold at auction for more than 20m Swiss francs ($21m;£13m).

The 76-carat Archduke Joseph diamond, described as internally flawless, comes from India's famous Golconda mines.

The diamond was sold to an anonymous bidder by the Christie's auction house in Geneva.

The price was well above the $15m (£9m) pre-sale estimate, and was far in excess of the $6.5m (£4m) it reached when it was last sold in 1993.

"It is a world record for a Golconda diamond and a world record price per carat for a colourless diamond," Francois Curiel, director of the international jewellery department at Christie's, told reporters.

"The market is not on the best form at the moment. The sale tonight was almost flabbergasting."

The diamond's seller, American jeweller Black, Starr & Frost, said they thought it would be "going to a museum", Reuters news agency reports.

Christie's specialist, Jean Marc Lunel, said the Archduke Joseph is prized because it boasts the highest quality of shape, colour and clarity that can be found in a diamond.

And it has an illustrious history.

The diamond came from the ancient Golconda mines, where the famous Koh-i-Noor and blue Hope Diamond originated.

It was named after Archduke Joseph August of Austria, a prince of the Hungarian line of the Habsburgs, who reportedly deposited it in a bank vault 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 World War II," the auction house said.

Decades later it surfaced at auction in 1961 and again at Christie's in November 1993.