To counter jmog, money has not always been coins of rare metals.Though gold coins have certainly made the best money over the course of human history.
The island of Yap used limestone stones, called Rai stones, as money for centuries. Until David O'Keefe found himself shipwrecked near the island and ruined their economy with "easy money" as it were.
In Africa, they used glass beads as currency. Glass beads were cheap in Europe, so when Europeans showed up and realized they could come back with easy money, they did. They bought up resources from the locals, slowly flooded the economy with easy money, and it eventually collapsed. Leaving the natives with essentially nothing.