Sapphire Crystal Watch Glass

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In the 1980s Swiss watch all watches use resin fibers as the watch glass, but due to the low density and hardness, it was easy to get scratches, so now watches are using anti-scratches and high density mineral sapphire crystal glass. Sapphire crystal is the result of an artificially compressed synthetic crystal, the advantages are high density, and less prone to scratches. Mohs hardness is 9, just behind diamonds.