Synthetic Sapphire: A synthetic sapphire crystal or “glass” is actually not glass at all. It is a very hard, transparent material made of crystallizing pure aluminium oxide at very high temperatures. Synthetic sapphire has the same hardness as natural sapphire gemstones, but without the coloring agents that give the gems their various hues. When it is heated, the synthetic sapphire forms round masses, that are then sliced into pieces with diamond-coated saws. These disks are then ground and polished into watch crystals. (One reason sapphire crystals are relatively expensive is that the tools required to cut and polish this extremely hard material are very costly.) Sapphire (whether natural or synthetic) is one of the hardest substances on earth. It measures 9 on the Mohs scale, which is a system for rating the relative hardness of various materials. (Diamond measures 10, the highest rating.) Watch crystals made of synthetic sapphire are often marketed as “scratch resistant” or “virtually scratchproof” because they are very difficult - but not impossible - to scratch. Diamond can scratch them; so can man-made materials that incorporate silicon carbide, which, with a Mohs rating of between 9 and 10, is harder than sapphire. So, if you accidentally scratch your watch on a simulated-stone surface or a wall that incorporates some of these silicon carbide materials, it could scratch the crystal.
