A woman who bought a diamond ring for £10 at a car boot sale is about to sell it at Sotheby's for £350,000

Thirty years ago, a woman in Isleworth, west London, bought an "exceptionally sized" ring she assumed was costume jewellery for £10 ($13) at a car boot sale.

Now, after decades of wearing the ring daily, she's about to get £350,000 ($455,000) for it at a Sotheby's auction in July, if it sells for its expected price.

That's because the ring is actually a 26-carat, cushion-shaped white diamond from the 19th Century, according to the BBC.

Head of Sotheby's London jewellery department Jessica Wyndham called the ring a "one-off windfall, an amazing find."

According to the Evening Standard, the owners are "incredibly excited." "Anyone would be in this position, it's a life-changing amount of money," she said. "No matter what your background is or what your past experiences have been, it's going to revolutionise someone's life."

Wyndham added that the unnamed owner did not think it was a real diamond when she spotted it at the car boot sale, which took place at West Middlesex Hospital, because it was "in a 'filthy' mount" and did not sparkle.

"The owner would wear it out shopping, wear it day-to-day. It's a good looking ring," she said. "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."

She added that the cut of the ring was "slightly duller and deeper" than today's designs.

"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," she said. "Cutters worked more with the natural shape of the crystal, to conserve as much weight rather than make it as brilliant as possible."

However, when a jewellery told the owners the ring could be valuable, they took it to Sotheby's and discovered its true identity after 30 years.

"They came in with the idea that it might be real and they had no idea of its value," Wyndham said, adding that Sotheby's examined the ring and had it tested at the Gemological Institute of America.

"The majority of us can't even begin to dream of owning a diamond that large."

She added that the owner had been to a number of car boot sales over the years, but hadn't collected antiques or diamonds. The ring will be up for auction in Sotheby London's Fine Jewels sale on July 7.