the Seven planetary deities of mesopotamia were all associated with a metal. You could base the magic effect/ mythical folklore of the metal on its associated god.

for example
Tin, associated with Marduk, the chief diety of the gods. Perhaps tin is the metal you swear on and it is bad luck to swear when touching it.
Iron, associated with Nergal, the diety of the underworld, plagues and war. Iron prepared in the proper way can be used for healing after which the metal corrodes and blemishes. Corroded iron on the other hand is associated with desease, death and decay, and can be used to harm "pure being" such as angels

I am not sure what kind of magical metals are you looking for. Do you want something like mithril or unobtanium? Or something alchemical which is hard to come by, but not mythical.