Civilizations have been creating currencies since the dawn of history. A pure barter system always has deficiencies, not least of which is transporting goods over mountains, through valleys, and across oceans. Think of the human effort involved in a simple transaction—not to mention the manure abatement fees—on ox-drawn wagons. So, to make transactions a bit simpler, almost all civilizations have come up with currencies.
From China to Africa to the Americas, shells were used as a medium of exchange. Whether small cowry shells or larger conchs, gathered in bags or tied together to form belts, shells held value in ancient cultures. Most shells used for currency throughout the world were not large decorative items, but small, transportable, and rather unremarkable specimens. In some cultures, the time and effort spent on tying the shells together to form belts, necklaces, and tapestries contributed to their value. Shells were so predominant as a currency that some of the first real coins minted in China were bronze pieces cast in the shape of small shells.
Let's face it, it's tough out there being a chief, especially on a small Pacific island where the resources are scarce and the people are hungry. The only way to project power is by wearing a pimp-tastic feather cape that conveys authority. Feathers, then, became a prized commodity in many island societies of yore. Eventually, however, feathers themselves were being used very little in the decorative arts and instead became a mode of exchange. As recently as the 1970s, some small island communities were still using feathers bound together in plates in intra-island trading.
In Mesoamerican cultures the cacao bean, from which we get chocolate, was the primary form of currency in the pre-Columbian era. The small, rather plain looking little pods had a pretty regular exchange rate throughout Mayan civilization in which you could buy a tamale for a few beans or the pleasure of a lady's company for 10. The beans were so valuable that the unscrupulous went through the effort of making counterfeits by shaving down avocado pits. When the Spanish invaded, it took decades before they appreciated the delights of chocolate. They preferred the silver and gold of Central and South America for their currency.
OK, to be fair, wampum could be lumped into the "shells" category, but it wouldn't have told the whole story. Wampum is a general name for a variety of shell beads that were produced by Native Americans from the 16th to the 18th centuries. Between various tribes and groups, wampum was exchanged to commemorate treaties, marriages, and grand occasions. The beads were often tied together in intricate belts that created pictures, told stories, and basically acted as a memory aid in the oral tradition. It was not until contact with the Europeans that the beads actually became currency unto themselves. Some of the colonies strictly monetized the beads and created a specific exchange rate between wampum and Dutch stuivers, or Spanish reales. Eventually, Dutch colonists started more industrial manufacture of the beads and wound up demolishing the market with overproduction.
Sometimes you don't have to look back in history to find examples of interesting currencies. The abalone, a rather rare mollusk, has been poached almost to extinction. Like many endangered creatures, its value as a food source greatly outweighs its actual tastiness. Its shell is valued for its iridescent shimmer that is a primary source of mother of pearl. The gastropods are so valued in Asia, however, that Chinese gangs are reported to be using the chewy creatures as trade for shipments of drugs, guns, and other contraband material.