The BBC show is believed to have made the startling discovery while filming at the Black Country Living Museum yesterday.
The experts are said to have looked at a Fabergé flower brooch, which has reportedly been valued at up to £1 million.
Executive producer Simon Shaw told the MailOnline: “We’ve had one of the most significant jewellery finds in 40 years of Antiques Roadshow history.”
However, details are being kept under wraps, with him adding: “But we don’t want to spoil the surprise.”
The delicate accessory was valued by Geoffrey Munn and was said to worth the staggering sum, making it one of the most expensive items in the show’s history.
Presenter Fiona Bruce was among those at the attraction, who revealed: “I’m excited that we’ve had such a good find today.”
Among the past priciest pieces was a model of Antony Gormley’s Angel of the North and the FA Cup, which was first used back in 1911, both valued at seven figures.
More recently, a Sir Lawrence Alma-Tadema portrait of engraver Leopold Lowenstam was inspected and Rupert Mass said it was “one of the best ever seen”.
It was shown after the subject’s great-great grandson took it along to Norwich’s Arley Hall.
While it was revealed the artist holds the record for a Victorian painting at £27.4 million, the smaller example was valued at a respectable £200,000 to £300,000.