1.I’m sure it’s glass - when I clean mine with isopropyl alcohol they make that ‘squeaky’ noise you get with glass rather than plastic, and they’re more resistant to scratches than most plastics are. According to manufacturer website K & F use German optical quality glass so I’m sure they would have been called out for that a while ago if it wasn’t true
2.I drop one last year and it broke like glass ..I’ve found them easy to clean and well made.
3.Looks and sounds like glass to me
4.HD optical glass
5.As far as I can tell it is glass. Excellent optical qualities