Sandpaper can scratch sapphire, as it is made of particles of carborundum. Same with some stone garden furniture. Sapphire is quite brittle: it has lower shock

Sandpaper can scratch sapphire, as it is made of particles of carborundum. Same with some stone garden furniture. Sapphire is quite brittle: it has lower shock resistance than glass, that's why quite expensive sports watches use some kind of glass, or plexiglass (famously, the Omega Speedmaster selected for the Apollo had a hesalite crystal).