Tiny sea shells once functioned as a working international currency across Africa

Cowrie shells harvested in the Maldives and Indian Ocean were traded into West Africa's Mali and Songhai empires by the 14th century, serving as currency, a unit of account, and a tool for paying taxes. Atlantic traders shipped billions more shells to the region to buy enslaved people and goods, and cowries stayed in everyday use in parts of West Africa into the 20th century.

— Wikipedia contributors, Shell money — West African use documented from the 14th century by Ibn Battuta