A ring bought at a car boot sale for £10 30 years ago because it was believed to be costume jewellery, is now expected to fetch £350,000 at auction, reports the Daily Mail.

The 26.27 carat, cushion-shaped, white diamond, was purchased at a sale at the West Middlesex Hospital in Isleworth in west London in the 1980s, and was presumed not to be real because 19th Century diamonds were not cut to show off their brilliance like today's gems.

It is going under the hammer at Sotheby's in London in July and their head of London jewellery department Jessica Wyndham commented: “The owner would wear it out shopping, wear it day-to-day. It's a good looking ring. But it was bought as a costume jewel. No-one had any idea it had any intrinsic value at all.”

Eventually the ring was brought into Sotheby's after a jeweller told the owner it could have substantial value.