A 26-carat diamond ring just sold for almost $850,000 at a London auction Wednesday and, while that’s certainly no record for expensive jewels, this particular diamond in the rough was first bought at a flea market for just $13 in the 1980s.

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The buyer, who asked Sotheby's auction house to remain anonymous, thought the gem was a piece of costume jewelry.

The auction house estimated the ring's value last month at close to $450,000 and it sold for almost twice that.

The ring was originally cut in a cushion shape by 19th century standards, according to Sotheby's, without the brilliant cut of today's diamonds. So it would have been easy to mistake it for a fake stone.

The buyer has not been identified.