Shopping at garage sales can be hit or miss, but one lucky customer is cashing in on a major find. 

Southeby’s will auction off a sparkly cocktail ring a woman bought at a car boot sale in 1980s England next month with a starting bid of £250,000 (roughly $325,000). She had thought for decades that it was costume jewelry and wore it without a care in the world, revealed the owner, who declined to be named publicly, the auction house said this week.

The woman had purchased the piece for £10 at a garage sale at West Middlesex University Hospital in Isleworth in west London, according to Sotheby’s. She decided in recent months to have the ring appraised after a jeweler said the stone could be real. 

Sotheby’s Jewelry in London confirmed the stone’s authenticity ― a 19th century 26.27-carat, cushion-shaped white diamond. The auction house added that the stone has a color grade of I and an impressive clarity grade of VVS2. The ring could go for as much as £350,000 (about $450,000).

“We confirmed that it was indeed a diamond. We got it tested with the Gemological Institute of America … and that then dictated the price,” said Sotheby’s Jessica Wyndham, who heads the auction house’s jewelry department.

In the interview in the video above, Wyndham added that it was understandable the owner did not know the ring held a real diamond. 

“It was in an antique style mount, so it was quite heavy,” she said. “It is mounted in silver around the top and when silver becomes tarnished it becomes quite black. And that mixed with the cut of the stone probably would have meant that it didn’t sparkle very much, and if it was all dirty, you just wouldn’t think that was real.”

The ring will hit auction on June 7.

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