A British woman purchased a diamond ring worth nearly half a million dollars…for almost nothing.

Now, she’s about to learn the true value of her purchase.

The woman purchased the ring in the 1980s at a car boot sale for £10 (about the equivalent of $10-15 USD). In the United Kingdom, boot sales are popular events in which people sell random knick-knacks and collectibles from their car trunks (otherwise known as boots).

But the original owner had no concept of the ring’s value. That was probably because of the relative dimness of the stone; old diamonds weren’t cut as brilliantly as modern diamonds, so the woman assumed that her ring was merely a piece of costume jewelry.

The stone had been cut in an antique cushion shape, which doesn’t allow for as much light reflection.

When it was made, its designer opted to preserve the natural state of the diamond, rather than make it more reflective and bright (and therefore much smaller). This was a common practice in the 19th century. 

As such, the woman thought of the ring as worthless; she was more excited for the designer clothes she’d purchased at the same car boot sale. She wore the ring regularly, as she liked the way it looked, but she didn’t think much of it. In fact, she ignored the precious stone for decades.

That changed when a jeweler told the woman that the stone might be valuable, so she decided to take it to Sotheby’s, which in turn took the ring to the Gemological Institute of America for testing. That’s when she learned what she’d been wearing for so many years: a 26.27-carat diamond.

She has decided to auction the ring via Sotheby’s, where it’s expected to sell for about £350,000 (about $450,000 USD) in June.

“The owner would wear it out shopping, wear it day-to-day,” said Jessica Wyndham, the head of Sotheby’s London jewelry department. “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 had been to quite a few car boot sales over the years,” she added. “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.”

“The majority of us can’t even begin to dream of owning a diamond that large,” she said.

The owner doesn’t want to be named, according to The Guardian. However, Wyndham says that the woman is “incredibly excited.”

“It’s a life-changing amount of money,” Wyndham said. “No matter what your background is or what your past experiences have been, it’s going to revolutionize someone’s life.”

Many diamonds have little or no resale value, but a natural diamond of this size will certainly find a buyer, according to Sotheby’s.

“The older stones have quite a bit of personality,” Wyndham said. “They sparkle in a different way.”

And for one lucky woman, that sparkle is life changing.