Thanks for the thorough reply, Yanagi! You explained it all pretty well as far as I can tell.

Yeah, it's a little disappointing, but it is what it is. I suppose I could always refer to the planets associated with each metal and attribute certain properties associated with the planets and/or the gods they're named after to the metal itself. Or just make something up, as seems to be the case with silver and werewolves (although there is a loose connection there, both are associated with the moon).