The lady owner snapped up the diamond, emerald and ruby brooch in Ohio before leaving it in the bottom of her handbag for months.
On a chance visit to a jeweller, she pulled out the brooch and shared the story of how she got it.
In a shocking revelation, the jeweller told her that the gems were the real deal and this was later confirmed by experts at The Gemological Institute of America.
The diamond in the rough features a 1.39 carat old mine-cut sparkler, a rectangular-shaped Colombian emerald which weighs 1.50 carats and an oval-shaped Burmese ruby, weighing around 0.60 carats.
US jeweller Bonhams will be selling the brooch at its New York jewellery auction later this month at a guide price of around $30,000 (£22,000).
Susan Abeles, head of jewellery, Bonhams US, said: “This is a really wonderful story and it just goes to show, treasures are really everywhere.
“What makes this brooch particularly impressive is that it features gem quality stones.
“For example, the brooch includes an old-mine scissor cut diamond – these scissor cut diamonds have old world charm.
“These kind of discoveries do not happen a lot so we are always excited to see how much the piece will achieve at auction.
“We expect it to catch the eyes of discerning collectors and jewellery lovers and I have a feeling it won’t end up in a garage sale again.”