The Micronesian island of Yap doesn't have any interesting rocks. Sedimentary in origin, its prosaic, shale strata don't offer an aesthetic challenge to the island's pristine beaches and tropical forests. So when Yapese sailors discovered limestone caves several centuries ago on the 200-mile distant island of Palau, they were naturally very excited, and decided to bring some pretty white stones home to impress the neighbors.
The first stone was carved in the shape of a fish, but this proved extremely difficult to get around. People wanted more, and a plan was devised: the Palauan limestone would be carved into discs with holes through the center, which could be rolled or carried by men with poles.
Limestone quickly became to the Yapese the sort of exotic luxury that gold and diamonds are to most of the rest of the world. The discs took years to carve, and the journey to bring such massive objects back to Yap in wooden canoes through the rough seas between the islands was an extremely dangerous one. Many boats sank. Sailors drowned, and hundreds of unwieldy rocks sunk to the ocean floor.
Human nature being what it is, the danger only increased the demand. Every village chief wanted a bigger stone than his rivals, to prop up outside his house in an ostentatious display of wealth. Being the most valuable commodity on the island, these giant imported stone cartwheels soon began to function as currency. A small stone could settle a debt or make a very generous gift. Bigger ones could buy a house or a wife, and often were used ceremonially to forge political alliances. The largest, more than eight feet in diameter, might be valued greater than an entire village. But size alone wasn't an absolute measure of value. A stone brought home on a particularly dangerous voyage would be worth much more than one of equal size that made its way through calm seas. The Yapese could tell the difference - many of the stones had (and still have) individual names.
Around 1878, Irish-American captain David O'Keefe was shipwrecked on Yap. Discovering the local demand for these strange objects, he decided to use modern, European nautical and stonecutting technology to produce and transport the stones more efficiently, making a fortune in the process. Cut with metal tools, O'Keefe's stones were smoother and shinier than those the natives had crafted with primitive hand tools. They were bigger, too, because they could be. Carried on a regular ship, the newer stones reached diameters of up to twelve feet.
It's now possible to synthesize diamond from carbon at high temperatures and pressures in industrial laboratories. Most diamond-tipped and edged tools are made from this material. It's every bit as functional as naturally-created diamond, and a lot cheaper. That hasn't put De Beers out of business, though. The diamond cartel still digs up gemstones at great expense, because people don't just want pretty things - they'll pay far more for rare things.
The Yapese reacted in the same way to O'Keefe's mass-produced money. Though bigger, the new stones didn't represent the effort and risk that made the earlier ones so valuable. The sea captain flooded the market, but he didn't devalue the local currency entirely. An older, entirely Yapese coin is worth far more than one procured with O'Keefe's help.
More than a century later, the official currency of Yap is now the United States dollar. Stone discs are still legal tender in the villages, though, where the people maintain a relatively traditional lifestyle. Over 2000 of them exist, on display for all to see: some outside houses, where they remain heirlooms and give the occupants great prestige. Others lie in public areas. Never stolen, they occasionally change hands, remaining in the same location and often being grown over with vegetation. The stones even have a "bank" of sorts, a canal in which most are stored.