Before I got to the end of your comment, I was just about to ask if it's possible to make a diamond coating. The scratch resistance would be very valuable, and I'm guessing if it's very thin, the chromatic dispersion wouldn't be an issue. This would probably be valuable for other applications too. However, I don't see how this would work on today's predominantly plastic (polycarbonate) eyeglass lenses, and switching back to glass lenses would make eyeglasses much heavier. It'd be great for smartphone screens though.