Watch Crystals
Here are some colored glass crystals, full picture below. Until about the 1940s, mineral glass was used to make watch crystals. At this time, plastic (plexiglass) crystals began to be used, as they did not shatter like glass. They were much more durable in that respect. Plastic can scratch, and can get yellow with time (though I think this yellowing is more an issue of the older forumlations). Scratches can be removed by the non-professional by a careful but not terribly complicated processing involving a polishing cream and a very soft cloth. Mineral glass crystals don't scratch as easily as plastic, but they can scratch, and when they do it's there pretty much forever. Most of the new production high-end watches use sapphire glass crystal, which is said to be very resistant to scratching and breakage.

I bought these colored crystals, because I thought they looked impressive and unusual. I don't know if these ever really were "the rave," but apparently somebody was manufacturing them. I'm not sure you'd really want a colored crystal, no matter how pretty, as it would darken the dial of the watch, and make everything monochomatic. I bought these sort of naively; it turns out that absolutely none of these crystals fit any of the watches I own. So now I have these things laying around, and I have no idea what to do with them. (Postscript: these watches have since been sold to a woman who refused to tell me what she planned on doing with them. She was not interested at all in watches and it remains a bit of a mystery to me.)