Watch glass, that is, the mirror material is divided into sapphire glass, mineral glass, synthetic glass three categories:Sapphire glass: the physical and chemical properties of this watch are very stable, and the hardness is very high. Second only to the diamond, can easily deal with scratches, wear, erosion. Many high-end watches will use this table mirror!Sapphire is the main chemical composition of aluminum oxide, high hardness, hardness up to Mohs 9, second only to diamonds, the hardness is almost 3 times the mineral glass, acrylic mirror 20 times. Sapphire shock resistance, sealing and hardness than other materials, the high cost of processing, so more in the top watch brand appears.