The "crystal" is the clear window on a watch that covers and protects its dial.
Protect your investment and upgrade from hardened mineral glass to a sapphire crystal. Synthetic sapphire is a hardened material that most luxury timepieces use due it being one of the hardest substances on earth.
If you want to nerd out on synthetic sapphires:
It is 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 agents that give the gemstone its various hues.
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.)