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So instead of copper, silver, electrum gold (the D&D standard), the early Roman Empire used various iterations of Bronze, Orichalcum, Silver, Billon, and Gold.
And today I learned Billon is the name for an alloy of silver and gold, or silver and copper, or silver and gold and copper, or basically any alloy of silver and some base metal.
https://en.wikipedia.org/wiki/Roman_currency#Imperial_period:_27_BC_%E2%80%93_AD_476