Discussion of sapphire glasses for smartphones on Habrah:
GreatRash: Corund also has the formula Al2O3, and the hardness is 9. The sapphire glass probably sounds just better than the corundum.
k12th: Natural corundum and sapphire (and ruby) differ only in impurities that make a difference in color.
Vanfukov: The sapphire attacks with cold, and the ruby with fire.