A diamond ring which was bought for £10 decades ago is expected to fetch £350,000 at auction.
The owner, who does not want to be identified, bought it at a car boot sale at West Middlesex Hospital 30 years ago, believing it to be a piece of costume jewellery.
It turns out the stone is a 26-carat white diamond from the 19th Century, and is likely to fetch hundreds of thousands at Sotheby’s in July.
After wearing the ring daily for 30 years, the owner took it to to the auctioneers who had it tested at the Gemological Institute of America and found out the true value of the gem.
The “dull cut” of the stone and its “filthy” mount led the owner to believe that it was not genuine.
Jessica Wyndham, head of Sotheby’s jewellery department, said: “With an old style of cutting, an antique cushion shape, the light doesn’t reflect back as much as it would from a modern stone cutting. Cutters worked more with the natural shape of the crystal, to conserve as much weight rather than make it as brilliant as possible.”