The Sierra Cup I have, and still use, from the early 70's is a dull flat gray metal (a little lighter in color than a photographic "18% gray card"). I assume it's aluminum or a non-stainless steel. Whatever it is, it probably only cost me a dollar back then and it's still going strong. No rust, no difficulty in cleaning. I also have a stainless steel cup that fits on the bottom of a Nalgene bottle. It holds more volume than the much flatter profiled Sierra Cup, but the Sierra is still my favorite - many memorable trips with that cup dangling from my backpack.