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.
Interestingly, local dispensaries at one point entertained the idea of BTC ATMs to get around the fact that they have no ability to take credit cards due to the illegal state of pot at the federal level, but this was abandoned after a bit because the volatility was too expensive to make this a worthwhile proposition.
I keep a small float in FIAT so I only do this when it’s the cheaper option.
In theory, what is stopping any stock from being a cryptocurrency (minus the crypto part)? Can anything tradeable with a fixed supply become a currency? What other magic requirement is missing?