Like I said, alumina is also called emery. As in emery boards. It's the stuff we use to file our nails. It's a common but very hard substance, and that means that alumina-based glass and ceramics can be much stronger and more durable than silica-based glass and ceramics, which is why it's useful for things like vehicle armor and durable watch faces.
Apple is a cheap ass then for not using it on the original iPhone/ iPad. Just think of the number of people with cracked screens that this could have helped avoid if it is as common and old technology as you say.