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Sapphire, Mineral, and Plexiglass (typically called plastic) are the three main crystals used on watches today.
Plastic is the least expensive and commonly found on vintage watches and many modern watches. These crystals scratch easily, however they are cheap to replace and easy to buff scratches out. Sapphire is the most expensive and the most scratch resistant. It can only be scratched by diamonds and other surfaces with a mineral hardness of 10. They are generally over $100 to replace and basically impossible to buff any scratches out. Since they are so hard, they are more likely to shatter on heavy impacts than a plastic crystal. A mineral crystal is between plastic and sapphire in cost and scratch resistance. It is virtually impossible to tell the difference between mineral and sapphire without taking a steel knife to the crystal to test it (not recommended!).