Sapphire is the second most scratch-resistant material known, after diamond, of course.
IOW, only a diamond will scratch sapphire. Or possibly also other aluminum oxide materials, I’m a little vague on that point.
I believe aluminum oxide is harder. That is why they use it for sand paper. AL/OX is also used in the 50 year warranty wood flooring finishes. Sapphire may be the second most for a natural material.