Watch crystals mainly are either mineral or synthetic sapphire. Mineral crystals are less expensive and can scratch somewhat easily. The more expensive sapphire crystals are near impossible to scratch but can chip on the edges. Acrylic crystals were prominent in the past and are mainly found on vintage watches. For sport models being used outside in the sun, an anti-reflective coating on the crystal is advantageous.

There is no such thing as a water proof watch, but some watches are more water resistant than others. Engraved on the case back of a watch, or on the dial, the level of water resistance should be stated. The level of water resistance can be expressed in feet, meters or ATM (atmospheres, also designated by BAR). ATM or BAR = 10 meters or 33 feet. The following is a chart of what different water resistance levels suggest.
- Water Resistant with no depth designation: OK for splashes of water or rain. Not suited for bathing, showering, washing dishes, swimming or diving.
- 50 meters (165 feet): OK for swimming in shallow water, bathing, showering and washing dishes. Not to be used for diving, snorkeling or other water sports.
- 100 meters (330 feet): OK for swimming and snorkeling but not scuba diving.
- 200 meters (660 feet): OK for skin diving (without scuba gear).
- 200 meters with a screw down crown: OK for scuba diving but not professional deep water diving.

If you see any evidence of moisture in your watch have it serviced immediately. Once water is inside a watch it will cause rust on the movement as well as the dial. The longer you wait the more expensive the repair can be.