A huge diamond ring bought for £10 at a car boot sale in west London is set to fetch £350,000 at auction.
Bought in the 1980s at a Sunday sale at West Middlesex Hospital in Isleworth , the 26.27 carat, cushion shaped white diamond piece will go under the hammer at Sotheby's in July.
For more than 30 years, the owner, unaware of its value, in fact thought it was a costume jewel and wore it while doing everything from shopping to chores.
The "exceptionally-sized" stone was thought not to be real as 19th century diamonds were not cut to show off their sparkling brilliance, as they are today.
The auction house's head of London jewellery department, Jessica Wyndham, said: "This is a one-off windfall, an amazing find.
"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."
The owners, who do not wish to be named, bought it into Sotheby's after a jeweller told them it could be of substantial value.
"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 said 'I think that's a diamond' and we got it tested at the Gemological Institute of America.
"The majority of us can't even begin to dream of owning a diamond that large."
The couple are "incredibly excited" as it's a life-changing amount of money.
The history of the diamond and how it arrived at the car boot sale is unknown.
It is the older style of a "slightly duller" diamond cutting which "could trick people into thinking it's not a genuine stone."
"The diamond cutter is looking to maximise the amount of brilliance from the stone, because that's what makes it sparkly," Ms Wyndham said.
"Serendipity may have prompted the owners to buy the ring in the first place.
"But a 26 carat, cushion-shaped diamond, no matter how filthy or dark that mount was, maybe that stone still speaks to people.
"They also bought some designer dresses for about £10. We should have all been at the car sale."
The diamond ring will be part of Sotheby London's Fine Jewels sale on July 7.
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