In this dawning age of digital and crypto currencies, it's always a wonder to think about what money is and what it isn't. It used to be that if you said the word "coin," what would instantly come to mind would be a small, flat metal disk with some sort of engraving or impression on it. Now, it can be data stored on a blockchain that has no physical form what so ever, and this money -- like Steem -- can flow to and from wallets that also do not have physical forms. However, our common conceptions of what a "coin" actually is has never always universally a small piece of metal. For example, the Micronesia island nation of Palau usued to have large, have coins made of stone.
If you think about this, the concept of money is so bizarre. You can't stick this into your pocket. However, there are other seemingly "unconventional" types of money. In ancient China, before imperial unification, there were like several different types of coinage -- one for every kingdom. I learned about this, once, because I in a very small park area beside a canal in Changzhou. I saw all of these weird symbols carved in stone. I took pictures and showed them to Chinese friends. Here are those pictures.
And here is a few scenic shot of the nearby canal.
(Image Credits: Everything but the above mentioned Wikipedia image is mine)