Sapphire crystal is the third hardest transparent material in the world - after diamond and moissanite. However, sapphire crystal is not actually a glass, but a crystal. To be more precise: a synthetically produced corundum.
Sapphire glass in a wristwatch is characterized above all by its scratch resistance. You should especially keep an eye on finely curved sapphire crystal. The curvature not only appeals on an aesthetic level, it makes the durable watch glass even more resistant. It's virtually impossible for your sapphire crystal watch to get scratched in everyday life - you'd have to try a diamond and very high pressure. Still feel like your scratch-resistant watch has been affected? It could be fine traces of aluminum that rubbed off when it came in contact with your watch glass. Just run an eraser over the glass and the "scratches" will disappear by themselves.