Yes, very much so (including something with unpredictable supply). Through history, diverse things were used as currency including shells, salt, silver and gold.

The commonly used definition of a currency was coined by Aristotle and is that it's an asset fulfilling three requirements:

- being an intermediary of exchange;

- being a store of value;

- being a unit of account.

You might notice that most cryptocurrencies are not actually currency owning to the first point.