Jill Cousins, from Market Harborough, saw pictures of her brooch on the long-running BBC One show, understood to be a favourite of popstar Lady GaGa.
On the programme, jewellery expert Geoffrey Munn was showing sketches of what he called his "most wanted" items. He estimated that the brooch owned by Cousins, made by the Victorian designer and architect William Burges, was worth about £10,000.
Cousins was invited to appear on Antiques Roadshow in Birmingham after she took the brooch to auctioneers Gildings for a proper valuation.
She was then informed that if she had sold the piece as planned "perhaps for £10" she would have lost out on thousands of pounds. Cousins had only avoided selling the brooch on the market after "she forgot to put it into her handbag".
Gildings said that the silver brooch was probably produced by Bruges for the wedding of his friend John Pollard Seddon and his fiancé Margaret. It is engraved with the initials "J.P.M.S." and features four turquoise forget-me-nots with a central red garnet heart.
Mark Gilding said: "It was all extremely exciting and Geoffrey Munn was overwhelmed to see something for which he had been looking for so long.
"The brooch had been inherited by Mrs Cousins's mother from her old primary school teacher 20 years before she gave it to her daughter. Neither of them liked it very much at that time.
"William Burges is such a highly regarded designer and architect, anything by him creates tremendous interest and items of jewellery are extremely rare. Now we hope it will attract great attention when it is offered for sale."
Last December, an Australian man claimed to have spotted his missing wife on Antiques Roadshow, almost 30 years after she mysteriously disappeared.