Since we are post-cataclyst, how about Steel coins, like in Dragonlance?
The more the world recovers, the more we re-open mines and the like, and the economy can gradually phase in precious metals.
Also, I like the idea of having some sort of currency and the relation between the different nations. However, by doing that, you are including the inflation concept in your game, and trust an old EU3 player, you are not sure you want inflation.
Hmm.. I guess you could designate a ressource you are producing to be the main currency. Like, if you are mainly a weath-producing country, you could designate the currency to represent a quantity of that weath (like the Koku of rice).
Never forget that all currencies developped based as a firm value of something. The Dollar originates as a value for silver. In a game like Elemental, you could have Orc Teeth, mana dust, or the Melange as a currency.