The most robust watch crystals are made out of sapphire glass. Only diamonds and carbide is harder and can scratch sapphire glass. Of course there are other materials such as some metals and stones which will scratch your sapphire glass crystal, so it is not wise to expect the watch to handle any abuse without getting marked. Sapphire has one downside though, it is highly reflective. Some watches receive an anti-glare coating to compensate for this, but sadly this coating is less hard and will scratch more easily.

Water tightness or impermeability, surprisingly enough, is the degree to which you can expose your watch to the element water. A watch can be water repellent to a variety of degrees, from surviving the odd splash, to keeping the watch fully functioning long after your crushed body has hit the ocean floor. Please note that these are always theoretical values. Water behaves very differently at different temperatures and with varying salinity. So we took our crayons and made you this rough guide (which, by the way, is in no way legally binding and we will not be held accountable for your watch if you ruin it):



FYI: 10m is 1 bar is (roughly) 33 ft is (exactly) 0.986923267 atm