The Least Practical Physical Currency

And no, I'm not talking about the money from Harry Potter, but seriously J K Rowling, having 29 Knuts to a Sickle and 17 Sickles to a Galleon is a prime-tastic mess of a system.

There is a small island called Yap in the Pacific Ocean which is home to the Yapanese people. They use large limestone rocks called Rai Stones as the local currency that look like this:

Since there was no natural limestone on the island, the locals valued it highly. So highly in fact that these Rai stones became used for trade. However, their size was often upwards of 8 feet across and so they were very difficult to physically move. Instead, when a deal was agreed the ownership of the limestone would transfer to the vendor, but it wouldn't actually be moved. The only thing that would have changed would be in the minds of those that had carried out the transaction: they would know that the ownership had switched.

As you walked around the island you would pass hundreds of these rocks laying by the side of the road or near people's houses and it was only through oral tradition, rather than geographical location, that their ownership would be known.

Some Rai Stones were worth more than others based on size and quality

One particular stone was being transported to the island by an outrigger and a canoe when it was accidentally tipped overboard before immediately sinking to the bottom of the ocean. It was agreed that although the stone rested out of human reach it was still owned and could still be used for trade. All that mattered was that everyone still knew the ownership, not that it could be produced on demand.

So, all in all, the system works when you have a small enough population that all the ownerships can be collectively remembered. However this whole system has strong parallels with our modern currency now that we deal with money which is not backed by the gold standard. Your bank notes are simply I O U notes that have value because we believe they have value.