On the banks of the River Thames lies a hidden treasure – Thames garnets. These deep purplish-red stones are a mystery, as garnets are not native to Britain and are not mined in England. Some say they are remnants of industrial cleaning or a nautical disaster. Others speculate they are the spoils of smuggling in the 17th and 19th centuries. Unearthed by legally permitted mudlarks, the stones are cherished finds along the riverbank. How they got there remains a puzzle, with theories ranging from ancient trade routes to clandestine antics. The Thames garnets may keep their secrets forever, adding to the mystery and allure of these unique gems.
https://www.atlasobscura.com/articles/thames-garnets-mudlark