A diamond ring bought for £10 at a car-boot sale 30 years ago is expected to fetch £350,000 at auction.
The owner believed the "exceptionally sized" stone was a piece of costume jewellery when she bought it at West Middlesex Hospital in Isleworth, west London, in the 1980s.
Unaware it was a 26 carat, cushion-shaped white diamond from the 19th Century, she wore it daily for decades.
The stone goes under the hammer at Sotheby's in June.
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The head of the auction house's London jewellery department, Jessica Wyndham, said: "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. They enjoyed it all this time.
"They'd been to quite a few car-boot sales over the years. But they don't have any history of collecting antiques and they don't have any history of collecting diamonds. This is a one-off windfall, an amazing find."
Ms Wyndham said the owner - who does not want to be identified - assumed it was not a genuine gemstone because it was in a "filthy" mount and it did not have the sparkle of a diamond.
She added that because the older style of diamond cutting was "slightly duller and deeper" than nowadays "it could trick people into thinking it's not a genuine stone".
"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."
After about 30 years of wearing the ring, the owners took it to Sotheby's when a jeweller told them it may be valuable.
"They came in with the idea that it might be real and they had no idea of its value," Ms Wyndham said.
"We had a look and... got it tested at the Gemological Institute of America."
She added: "The majority of us can't even begin to dream of owning a diamond that large."