A diamond ring which was purchased at a car boot sale for £10 has been sold for £656,750 at auction.

The cushion-shaped jewel was expected to fetch up to £350,000 when it went under the hammer at Sotheby’s in London, but in the end was snapped up for close to twice that amount.

Named the “Tenner”diamond, the ring was originally purchased in the 1980s from a car boot sale.

The 26.27 carat diamond is thought to have been cut in the 19th Century but its history and how it arrived at the sale is unknown..

The owner of the ring had bought it as a costume jewel and worn it daily, before being told by a jeweller of its substantial value.

It is thought the stone is from the 19th century, when diamonds were not cut to show off their brilliance like today’s gems.

This results in an appearance slightly duller and deeper than modern diamonds, which could explain why the ring’s true value was ignored for so long.

It was recently confirmed by the Gemological Institute of America (GIA) as a genuine diamond.