An acrylic watch crystal is hardened plastic, which has the advantage of being resistant to shattering, but can be scratched fairly easily. Fortunately, scratches can be easily removed from acrylic.
A mineral glass watch crystal is hardened glass (Seiko name – Hardlex), which is much more scratch resistant than acrylic, but significantly less than sapphire. Mineral crystals are also somewhat better at shatter resistance compared with sapphire.
A sapphire crystal is made of synthetic sapphire, which is a transparent form of corundum, or aluminum oxide (Al2O3). It is extremely hard (Moh’s scale 9 – Diamond is 10), and will resist scratching by most substances short of diamonds. Premium watches generally use sapphire crystal (e.g. Rolex Omega AP Panerai Bremont Hublot GP Cartier).
Crystaltimes produces 4 basic designs of sapphire crystal.
1) Flat (both sides)
2) Single Dome – Domed top surface flat underside.
3) Double Dome – Domed top and underside surfaces.
4) Bubble Dome – A high double domed bubble like design.