Nothing is indestructable. Metal keys and such may not do a lot of damage, but sand and other objects can easily scratch. I notice this with my aquarium. I've never scratched it with metal scrapers and razor blades, but get a little sand between the magnet cleaners and it will easily scratch.
I remember as a kid certain ceramics can easily scratch or shatter glass. You could easily take out a car window flicking a pebble size piece of ceramic from a broken spark plug. Hard to believe, but it's true! So it's not the metals I'm worried about, but the quartz, mica's and ceramics that do the real damage.