Beau Sancy, one of the world’s oldest and most famous diamonds, has sold for $9.7 million at auction in Geneva – twice its reserve.

The 35-carat gem was worn by Marie de Medici at her coronation as queen of France in 1610.

It has since passed among the royal families of Europe, used to show off their wealth – and pay their debts.

Beau Sancy, one of the world's oldest and most famous diamonds, has sold for $9.7 million at auction in Geneva

The pear-shaped, rose-cut stone attracted fierce bidding and was won by an anonymous buyer.

“It’s extremely rare for a diamond of this importance to come on the market,” said David Bennett of auction house Sotheby’s.

“It’s never been out of royal hands, it’s of immaculate provenance, it’s in all the history books of famous stones.”

After being worn at Marie de Medici’s coronation, the diamond was pawned by Mary Stuart to finance the fight by her brother, the future Charles II, for the English throne, following the English Civil War.

Later still it was the centrepiece in the crown of the first king of Prussia, and it was sold by the Royal House of Prussia, the descendants of those who once ruled Prussia.

But, as its buyer was anonymous, no one knows whether it is royalty or commoner who owns the Beau Sancy now.