Famed diamond sold

One of the world's most admired diamonds has been sold at auction for 18.1 million Swiss francs ($US21 million).

The 76-carat Archduke Joseph diamond was sold to an anonymous bidder by the Christie's auction house in Geneva.

With the buyer's premium, the bidder will pay more than 20 million Swiss francs.

The BBC reports the diamond was last sold in 1993 for $US6.5 million.

"The market is not on the best form at the moment. The sale tonight was almost flabbergasting," said Christie's international jewellery department director Francois Curiel on Tuesday.

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

The BBC reports 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.

It surfaced at auction in 1961 and again at Christie's in November 1993.

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