It looks like a fairly universal currency: Roman soldiers were paid in it (at least, "salary" comes from the word for salt), as were soldiers in the American War of 1812. In Timbuktu they traded salt for slaves. It created cities and the Polish kingdom. It was taxed for hundreds of years in Europe.
These days salt is practically free, but when the only way to get it was from mining or by evaporating ocean water (neither very readily accessible in many parts), and unlike gold, you tended to consume it in preserving food, it was worth a bit more.
