The sale took place at the Bonhams New York Fine Jewellery auction on 19 September.
According to a Bonhams statement, the brooch was bought by an anonymous woman as a gift for her daughter about five years ago; however, the owner placed it in her handbag and forgot about the item.
“On a chance visit to a jeweller many months later, she pulled out the brooch and was sharing the story about how she acquired it,” the statement read.
“To her complete surprise, she was told the brooch was not costume as the diamonds were real. Bonhams was then approached and the fine jewellery team undertook some further detective work and confirmed to the client that not only were the diamonds real, so were the emeralds and rubies – and they were gem quality.”
The brooch, originating circa 1900, featured a rectangular-shaped emerald, an oval-shaped ruby, an old mine-cut 1.39-carat diamond, as well as old European-cut diamonds weighing an estimated 1.25 carats throughout. It was set in platinum and 14-carat gold.
Commenting on the headlining piece, head of jewellery for Bonhams US Susan Abeles said, "Discoveries like these don't happen very often, so we were delighted to offer this at auction where the actual value of the piece was finally realised."
More showstoppers
The top-selling lot at the Fine Jewellery auction was a step-cut 16.9-carat diamond ring that sold to a private US buyer for US$1.8 million (AU$2.2 m).
"This is one of the finest examples of a diamond there is given its rarity, colour, cut, clarity and transparency,” Abeles said of the sale.
Jewellery from the personal collection of Gigi Guggenheim Danziger, whose husband owned Cartier for four years, was also part of the auction, and comprised a number of signed Cartier pieces from the 1960s and 1970s.
Highlights from the collection included a diamond necklace featuring three rectangular-shaped diamonds set with round brilliant-cut diamonds that fetched US$37,500 (AU$47,800) and a ruby and diamond brooch.
The brooch showcased pear-shaped rubies and marquise-shaped diamonds and was reportedly given to Guggenheim Danziger as an anniversary present before she wore it to the London premiere of movie The Godfather. It sold for $US46,000 (AU$57,770).
According to the auction house, other strong results included pieces by French jeweller René Lalique and American designer Oscar Heyman.
The next Bonhams Fine Jewellery sale takes place in New York on 4 December.
Headlining items
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