6 ideas
Maldive cowries (Cypraea moneta) worked as currency because they were durable, uniform in size, hard to counterfeit, and impossible to produce locally in West Africa. Their low unit value also made them suited to small everyday market transactions that gold or cloth could not handle, so they filled a gap in the demand for small change.
Cowries were gathered in the Maldives and shipped to Bengal and Ceylon, often as ballast. Dutch and English East India companies then carried them to Amsterdam and London, where they were re-exported to the West African coast. A local shell became a global currency only by being routed through several separate trading systems, each adding its own costs and markups.
Cowries were a major import that European traders exchanged for enslaved people at ports such as Whydah and Lagos, and in some periods they made up a large share of the goods paid per captive. The currency therefore directly financed the Atlantic slave trade.
Each shipment of shells expanded the West African money supply, so the European traders who controlled the supply were in effect setting monetary policy for societies outside their jurisdiction. Seeing the import trade as control over the money supply shows how external suppliers determined local price levels.
In the mid-nineteenth century, German and French firms shipped huge volumes of the larger, cheaper East African cowrie (Cypraea annulus) from Zanzibar to West Africa. The unlimited supply sent shell values into a steep decline, and prices measured in cowries rose sharply.
Colonial governments imposed coinage, refused cowries for taxes, and eventually banned imports, which completed the collapse of a currency that inflation had already weakened. Transport and counting costs became larger than the shells' value, so cowries fell out of use. Holders of cowrie wealth lost savings built up over generations.