$15 'costume jewelry' ring turns out to be 26.27-carat white diamond worth over $450K

— -- One lucky lady snagged the deal of a lifetime over 30 years ago when she purchased what she thought was merely a fun cocktail ring, but what turned out to be a diamond worth more than $450,000.

The unaware buyer who spent approximately $15 at a flea market in London had no idea the gem was actually a 26.27-carat white diamond.

On Monday, Jessica Wyndham, head of Sotheby's London jewelry department, met with the owner who has been wearing the ring since the '80s and only recently found out from a local jeweler it could be of significant value.

"She first bought it in the 1980s as a costume jewel, cocktail ring and she has been wearing it around ever since," Wyndham said. "It's impossible to really date it, but the style of the diamond has notable characteristics similar to what you would expect from the 19th century," she explained.

"She randomly took it to a local jeweler who said, 'This could be a diamond,' and told her to 'seriously get it looked at,'" Wyndham added.

The jewelry specialist for the international auction house told ABC News that the ring's owner was searching impressive diamonds on Google when she found Sotheby's and reached out to Wyndham to get the process started.

Once Wyndham first saw the jewel in person earlier this year, she contacted the Gemological Institute of America to have the gem identified. "They check the diamond and give a certificate confirming the color, clarity, size and weight," Wyndham said of the New York City-based company.

Wyndham said she was astonished that given the weight, color, cut and clarity that an average person could own this kind of stone without knowing what it really was.

"Even if it was just a small 1-carat diamond it would be amazing, but the fact that it's 26 carats, that's larger than most would see in their lifetime let alone own dream of owning. To have one this big and truly pure is astounding," Wyndham explained.

Sotheby's will start the low auction price at about $325,000 but Wyndham said the diamond could reach $454,000. "We don't stop people from bidding past an upper price. The hope is that people will continue bidding, it's really just the low estimate that people should focus on," she said.

The unique 26.27-carat white diamond will be part of a sale on June 7, 2017, that includes a diamond broach worn by former British Prime Minister Margaret Thatcher and other priceless historic jewelry items.