Fine watches have either watch crystals made from synthetic sapphire, or tempered mineral glass watch crystals. Lower end watches may have plastic covering the watch dial. Mineral glass watch crystals are the typical watch crystal for a standard luxury watch. Outwardly, they are look the same as synthetic sapphire watch crystals but they can scratch much more easily (though not as easily as non-tempered glass or the plexi-glass used in non-luxury watches).
If your watch crystal is made from mineral glass, be very careful to keep your watch away from items such as keys, coins, tools, along with anything else that might scratch it. Store the watch in a soft cloth or in the case that it came in. Diamonds are the hardest natural surface, so be careful with your watch around jewelry that contains diamonds. Finally, mineral glass can shatter, so be sure not to drop it or allow it to absorb heavy shocks.
The highest quality Swiss watches use synthetic sapphire crystals. Synthetic sapphire is one of the hardest substances on earth. These crystals can sometimes be marked “scratch resistant”, because they are very difficult to scratch due to their hardness. Synthetic sapphire has a grade of “9” on the Mohs mineral hardness scale; therefore, only materials with a higher score should cause scratches. Diamonds are graded “10” on the Mohs scale, so use extra caution with diamond jewelry around your sapphire crystal.
If your watch crystal receives minor scratches, it may be possible for the crystal to be refinished back to its original condition. If the scratches are too deep, your watch may require a crystal replacement. If your crystal does need to be replaced, make sure that you use an authorized watch repair center that uses parts approved by your watch manufacturer.