Historic Beau Sancy royal diamond sells for $9.7m in Swiss auction

A CENTURIES-old diamond passed down through generations of

European royalty fetched nine million Swiss francs ($A9.77 million) at auction in Geneva.

The 35-carat Beau Sancy diamond was worn by Marie de Medici, Queen consort of Henry IV, at her coronation in 1610.

Five bidders spanning three continents competed for the historic jewel at a Sotheby's auction where there was little evidence of the current global financial woes.

An anonymous telephone bidder purchased the jewel, put on the market by the House of Prussia and described by Sotheby's as one of the "most fascinating and romantic" gems ever to come to auction.

The buyer paid 9,042,500 Swiss francs including the buyer's premium for the pear-shaped, double rose cut diamond - more than double the $US2-$US4 million estimate.

"You are buying an historic work of art - you are not buying a diamond," said Philipp Herzog von Wuerttenberg, chairman of Sotheby's Europe, following the sale.

"I fell in love with it when I saw it. It's the cut, it's the history," he said.

The Beau Sancy attracted bids from North America, Europe and Asia, he said, refusing to give further details about the buyer.

The diamond's royal connections date back to 1604 when it was bought for Henri IV of France at the insistence of his wife Marie de Medici who wore it atop her crown at her coronation.

Later that century it was acquired by the Dutch and used to seal the wedding of Willem II of Orange Nassau to Mary Stuart, daughter of Charles I of England.

Stuart pawned the rose-cut gem to finance her brother Charles II's fight for the throne.

In 1702, the first king of Prussia gave it pride of place in the new royal crown and it has passed through generations of the House of Prussia until today.

"We've sold much larger diamonds but it has this wonderful romantic history, an unparalleled royal history - it has never been in non-royal hands," said David Bennett, co-chairman of Sotheby's Switzerland, ahead of the sale.

The Beau Sancy went under the hammer at Geneva's Beau Rivage hotel as part of Sotheby's Magnificent Jewels and Noble Jewels sale.