There could be as much as £60 million worth of jewellery sitting forgotten in the UK's attics and cupboards, according to auction house Bonhams.
As tastes change, it says, items are often put aside and their value forgotten.
"We estimate there is at least £60 million worth of unwanted jewellery that owners have forgotten or never worn just waiting for a new home," says Jean Ghika, head of jewellery at Bonhams.
"A lot of people have no idea just how valuable these pieces could be."
Recently, for example, the company identified a dusty piece bought for £1.50 in a charity shop as a valuable Cartier ruby brooch. It later sold for £2,400.
Other finds include a piece of 'costume jewellery' given to a girl by her grandmother that turned out to be worth £12,500; she'd been using it for dressing up. Other examples include an inherited ring that was revealed as a rare Burmese unheated ruby and which sold for £134,500; and a £20,000 diamond ring that was found buried in a jar in a back garden.
The news comes as the company kicks off its annual Jewellery in June event.
"We're asking everyone to have a look in their drawers and jewellery boxes, dust off any items they haven't worn for several years or may not know much about and bring them in to any one of our regional offices during June," Ms Ghika tells the Daily Mail.
"The jewellery market is very robust at the moment and items such as natural pearls and good quality diamonds and coloured gemstones are achieving excellent prices."
As the Antiques Roadshow regularly proves, it's always worth having a rummage in the attic. We recently reported, for example, on the sale of a tiny £120,000 oil painting that had been sitting forgotten in a Dorset attic for eight years.
And it's not always easy for the amateur to tell which items of jewellery might be worth thousands. While a cultured pearl necklace may only be worth a few hundred pounds, a natural pearl one could be worth ten times as much.
Signs that a piece may be valuable include hallmarks or engravings; slightly irregular, hand-cut stones; or an original box or receipt.
"Current market conditions for fine jewellery, most specifically period pieces, fine coloured gemstones, diamonds and natural pearls, has never been stronger," says Ms Ghika.