You can indeed scratch a sapphire crystal. They call it sapphire, it's all corundum. So are the 'rubies' in the movement. Just depends whether you dope the crystal and with what. The only reason they don't use clear corundum in the movement is because it confuses people when they do.
It's just harder to scratch the sapphire cystal than it is to scratch lesser crystals. If you go scraping it against brick, there are plenty of things in bricks that can scratch it. Like more aluminum oxide crystals. But in casual use it will take less damage than glass.
Back in the day, when Rolex could be taken seriously, they used acryllic crystals on all their watches because while easily scratched, they were also easily polished, and they are far less likely to break than sapphire or glass.
fwiw, when they say "mineral crystal" they mean "not plastic" which generally means glass. Not all glasses are alike, and some crystal types, like seiko's hardlex, are quite scratch resistant.
