LONDON -- Some people have all the luck.

Consider the person who dropped about 10 pounds (about US$15) around 30 years ago on what was thought to be a piece of costume jewelry -- it turned out to be a 26.27 carat white diamond.

The gem bought at a flea market is expected to fetch about 350,000 pounds ($454,000) when it is auctioned by Sotheby's next month.

The buyer had no idea of the ring's value because 19th-century diamonds were not cut to show off their brilliance and clarity.

The auction house's jewelry department chief, Jessica Wyndham, said Monday the owner wore it on a daily basis, unaware that it was a real diamond.

She called it "an amazing find." The owners have asked not to be identified.