It depends on how you look at it. It is "natural" in that there are no chemicals in it's making. It is a combination of several elements which are mined. In the same way as bronze and brass is...
Stainless steel is an alloy of steel (which is iron with a bit of carbon) and the element chromium. The chromium content of stainless steel varies, but is generally a bit over 10% to give it the...