I have recently reinstalled some salvaged 50 feet of 12/2 MC. I installed my recessed can fixtures and began installing 6' pieces of said MC to their respective 4 squares. Upon stripping the MC I found that the hot and neutral are 12 gauge stranded copper and the bare ground appears and feels like a solid aluminum. Is this a common type of MC or something used for a specific application or am I just plain wrong and it's a tinned piece of copper? Any input would be appreciated. Thanks in advance.