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[goodcitizen]

An acrylic watch crystal is a kind of hardened plastic, which has the advantage of being resistant to shattering, but can be scratched easily. Fortunately, scratches can be easily removed from acrylic.

Mineral crystal is a kind of hardened glass, which is much more scratch resistant than acrylic, but less than sapphire. Mineral crystals are also somewhat better at shatter resistance compared with sapphire, especially in those cases where the crystal is domed.

A sapphire crystal is indeed made of synthetic sapphire, which is a transparent form of corundum, or aluminum oxide (Al2O3). It is extremely hard (Moh's scale 9), and will resist scratching by most substances short of diamonds. However, if struck sharply and from the correct direction, sapphire will shatter, especially if domed