Iron is a natural resource that can be found underground. It is used to produce iron bars.
Iron is often found in stone small veins of roughly 10-30 pieces - an easy way to find a small iron vein is to find a cliff face and search the rocky areas - you will often find one or two outcrops which can lead to veins.
It's also possible (but less likely) that you will find a mountain with a very large deposit of iron making up its center - yields of 10,000 iron are not unheard of.
One block of copper turns into one block of copper bars.
Added in the game-date.