$13 diamond ring assumed to be costume jewelry sells for nearly $850,000

A $13 stone assumed to be costume jewelry ended up being a precious stone. A $13 stone assumed to be costume jewelry ended up being a precious stone. Image 1 of / 1 Caption Close $13 diamond ring assumed to be costume jewelry sells for nearly $850,000 1 / 1 Back to Gallery

One lucky woman unknowingly bought a 26-carat diamond for $13. Wednesday it sold for nearly $850,000.

The enormous rock was purchased in the 1980s by an anonymous person who assumed, likely because of its size, that it was a fake stone at the center of the band. It turned out to be very, very real.

The ring sold for an astounding £656,750 ($847,667) at the Sotheby's Fine Jewels auction in London.

That's nearly double its estimated worth. It first made headlines in May when it as appraised at a whopping £250,000 ($325,000) to £350,000 ($456,000). (Story continues below.)

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"The new owner is likely to re-cut it into a modern diamond that will emit even more sparkle and potentially be worth a multiple of today's price," Tobias Kormind, diamond expert and managing director of 77 Diamonds, said in a prepared statement. "I'm convinced the £10 ($13) ring was once owned by royalty or a person of great wealth, because it originates from the 1800s -- before the discovery of modern diamond mines and a time when very few diamonds were available."

Part of its value-generating attributes is its unique cut and 19th century mount.

"When we think of diamonds, we think of modern cuts, of brilliance," Jessica Wyndham, head of Sotheby's jewelry department, told CNN in May. "This wouldn't have looked like that. The silver had tarnished and there was probably some dirt. These diamonds were made for candlelight, not our white artificial light, so it was all about trying to bring out its fire."