For me it is the fact that our blood contains iron. I earlier used to believe the word stood for some ‘organic element’ since I couldn’t accept we had metal flowing through our supposed carbon-based bodies, till I realized that is where the taste and smell of blood comes from.
Iron would be a terrible metal for coinage, since it would shed rust all over everything after being handled. Some coins might be cast from iron (if it’s cheaper than alternatives, idk) but plated in other metals to prevent that.
Only faerie-adjacent realms still use iron coinage, because it keeps the little fae bastards from picking your pocket.