Metal. FYI: some sandwich places have their sandwiches wrapper lined with aluminum.. I didn't know that and when I stuck my sandwich in the microwave to heat it up I caught my sandwich on fire... It did not end well. o_O
Small fluorescent lights will glow in the microwave. (If you turn it on). Try it at YOUR OWN RISK. I've never let one glow more than 3 or 4 seconds before stopping it, with no problems. If it can stay in there indefinitely, it could be a cheap substitute for cable, or at least a glowy spinny thing in your dark kitchen at night so you can find your snacks.