Stainless steel. A luthier friend up the street did it for me. He used to do custom metal work as decorative touches to guitars, but moved on to building guitars from scratch, often with decorative metal elements - copper, stainless, brass... kinda lost touch with him as he sold his building and moved to Morocco.
I had it done because I had subjected my guitar to plenty of abuse in the 80s and it was pitted and scratched something fierce. I suppose I could always take off the metal, strip what remains of the black and do a refin. But I kinda like it the way it is. Most of my stuff looks a lot tamer, so I enjoy the contrast.
That said, some of these pristine S-100S I've seen in here look so gorgeous all on their own, ya know?