If you have a 9 Moh screen protector on your smartphone, you have a good "test case" for durability of a Sapphire Crystal watch face. I carried my cellphone everywhere daily on the farms in an open holster with a one of those screen protectors on my smart phone/phablet. The upper part below the edge was marked when I looked at it at an angle in the sunshine but, I never saw it in actual use.
I carried this phone cutting firewood, working the "garden", working cattle, etc. My phone had a hard life and the screen protector came through it all unfazed. I suspect I would be more apt to nick the bezel than damage the Sapphire Crystal watch face.