True sapphire is very difficult to scratch. If you can feel the line with your fingernail but it doesn't appear to go deep into the stone, it may be a scratch in a protective film or a lower-quality synthetic material.
Fracture Analysis and Failure Mechanisms in Single-Crystal Synthetic Sapphire Components sapphire cracked
The primary fracture mechanism in sapphire is cleavage along specific crystallographic planes, typically the rhombohedral planes ${10\bar{1}1}$ and basal planes ${0001}$. When a crack initiates, it tends to propagate along these planes of weak atomic bonding, often resulting in smooth, mirror-like fracture surfaces rather than the conchoidal (shell-like) fracturing seen in amorphous glass. True sapphire is very difficult to scratch