Synthetic sapphire?
is the most expensive glass crystal material and the most scratch resistant. A very hard, transparent material made of crystallizing aluminum oxide at very high temperatures. Chemically, synthetic sapphire is the same as the natural sapphire used in jewelry, but without the coloring. Because it is so hard, it is also brittle, and shatters more easily than mineral glass or Plexiglas. Sapphire (whether natural or synthetic) is one of the hardest substances on earth. It measures 9 on a scale of 10. Diamond is a 10. Some crystals are made of both mineral glass and sapphire. Seiko, for example, makes some watches with crystals made of mineral glass covered with a layer of synthetic sapphire called “Sapphlex”.
