Type of metal used in a relief portrait I purchased.

Recently at a thrift store purchased a 6 inch relief portrait of a bearded man with a headband. I assumed it was copper, but when I got it home, I put a magnet to it and it stuck like glue. The coloring is very dark and it has tiny pock markings everywhere. Could anyone give me some thoughts as to the type of material it is made from? Greatly appreciated, and thank you for your time. Danny.