Pearls adorn Scotland's history from crown jewels to folk history.
But after centuries of fishing, Scottish freshwater mussels are now so rare they've been given legal protection.
Unscrupulous poachers are blamed for much of the damage, but fishing is not the only threat to the mussels.
Their ability to recover is hampered by polluted waters and painfully slow reproduction.
Jane Franchi visits the riverbank, hears the science and dips into the history of one of Scotland's great natural treasures.