Its nearly 3 am here and I just had a thought. Instead of having metal items require a specific metal, have them require just the generic form "metal", kinda like how string works. However, each metal has different properties, which means that wearing a cast iron plate would probably be worse than leather (its so cumbersome, it makes you slooow), gold being both soft and heavy is probably a really bad idea for armor and weapons too. I haven't thought out every application of the current metals on the items we have at the moment, but I think this would make the whole industry side of this game a bit more interesting.
I know that this would completely change the value of metals, but I think its a fun idea and am hoping y'all can help work out the kinks.