Have you had one of those fancy new five pound notes yet? I thought my first one was some kind of Monopoly money at first. It can take a little while to get used to a new form of currency. It may surprise you to find out that money hasn’t always taken the form of paper or even precious metal coins. In many places in the world, shells and even cows have been used as a form of currency for a long time.

Rai stone

Credit: Bartek.cieslak

But perhaps the most unusual form of currency in history belongs to the small island of Yap, which lies in the western Pacific Ocean, and is part of the Federated States of Micronesia.

A rai stone at the Currency Museum in Ottawa

Credit: Beades

For centuries, the people of this tiny island used large carved discs of calcite stone (called rai stones) sometimes up to 12 feet (4 metres) in diameter, which looked a bit like doughnuts. Luckily, most of them were much smaller than this, but they were all still pretty heavy. They were shipped from neighbouring islands, sometimes coming from as far away as New Guinea. As they were so difficult to obtain on Yap, they maintained their high value.

A rai stone in the village of Gachpar

Credit: Eric Guinther

As you can imagine, people didn’t tend to bother moving these heavy stone discs once they’d arrived (some of the largest would take 20 men to lift), so islanders simply kept track of who owned what stone, and this would allow the owner to trade for any goods and services they might need.

A rai stone in the British Museum

Credit: Ken Eckert

At one point, when a large stone was being shipped to the island, a storm caused it to come loose from the canoe and sink to the bottom of the ocean, where it remains to this day. You’d think this would be a bit of a blow to the owner, but you’d be wrong. The islanders might not be able to see the stone disc, but they knew it was there. So for many years, a rock that no one could see or touch, was used to barter and trade with other people.

Rather sadly, the island now uses US dollars as its currency. Which is a bit of a pity for whoever owns that underwater rock.