A woman who for decades wore what she thought was a piece of costume jewelry will reap a large windfall after the outsized stone in the ring was found to be a real diamond.

An enormous 26-carat white diamond, purchased for $15 as costume jewelry at an Isleworth flea-market sale in London in the 1980s, and mistakenly worn for years as a casual ornament, is expected to fetch over $450,000 at a Sotheby's jewelry auction on June 7.

The large stone in the dirty ring was thought to have been ignored by experts because 19th-century diamonds were cut in a different way than modern stones.

The lucky owner wore the ring for decades thinking it was an imitation, doing the chores, shopping at the grocery and going about her life.

As detailed by a spokesperson for Sotheby's jewelry office, "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."

The ring "is a one-off windfall, an amazing find," the spokesperson added.

After some 30 years, the owners, on the recommendation of a local jeweler, brought it to Sotheby's who subsequently sent it to the Gemological Institute of America, according to the Guardian.

Sotheby's suggested that the style of the stone's cut was "slightly duller and deeper than you would see in a modern style… it could trick people into thinking it's not a genuine stone."

"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 of the crystal rather than make it as brilliant as possible. The older stones have quite a bit of personality. They sparkle in a different way," the spokesperson added, cited by the Guardian.