Acceptability
Durability
Divisibility
Stability
Portability
(Elasticity)
If it spoils or decays within your lifetime it's not a currency.
If it's not something you can divide in order to make payments of a more-or-less arbitrary amount, it's not a currency.
If you can't predict how much of the asset you will need to pay your bills next month, it's not a currency
If you can't bring it with you to the place where the exchange takes place, it's not a currency.
If you can't obtain capital investment in a currency because the currency itself is more valuable than anything you could produce, then it's not a (good long term) currency. This last one is in parens because it one only matters in a growing economy. A deflationary currency can still otherwise function as a viable medium of exchange, but eventually, lack of availability for new entrants will mean that entrepreneurs will begin looking to do business in alternate currencies.