I just popped a movie on in the background, Wagon's East, it's a Western comedy set in the 1860's, so maybe this is a movie mistake.
But Robert Picardo (the StarTrek:Voyager Doctor), is wearing what I call a visor-hat, IDK the real name. It has a greenish semi-transparent material, that now-a-days would be plastic. But I'm sure I've seen there old-style hats in other TV/movies. Sometimes you see bookies, and accountants wearing them in old gangster movies.
So if they really had these see through visors in the 1860's, what the green material would be ? Besides glass, which seems a bit risky, maybe it could be some plant material, maybe died green. Or maybe even treated animal skin. I'm guessing it's not something like the mineral Mica.
I can't take a screen shot of the hat as the movie is on amazon. But here's a modern one.