For what it's worth, real-world coins before the fiat currency era were not pure silver or gold. The important part was that a coin was recognizable and had a standard size and metal content. Mercury dimes were 90% silver, 10% copper. (I'm surprised it was that close to pure, actually! Just looked it up.) So if I were you, I'd drop the idea of "GP,SP,CP" and give your coins names: shekels, doubloons, crowns, shillings, marks, or something made up.
I know that salt was once used as currency in Roman times. Hence the word "salary".