Sapphire

A sapphire watch case refers to a watch with a crystal or glass case made from synthetic sapphire. In the context of watches, the term “sapphire” usually refers to the sapphire crystal, which is used to protect the dial of the watch (not the entire case). However, is can be used as material for the case as well. As it is highly prized in watchmaking because of its durability, scratch resistance, and clarity.

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Measuring the hardness of minerals can be done with the Mohs hardness scale. Sapphire crystal is one of the hardest substances (9 on the Mohs hardness scale), just below diamond (10). This makes it highly resistant to scratches, unlike regular glass or acrylic, which can scratch easily over time.

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Hardness Vickers of sapphire

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Hardness Vickers is another scale of measuring hardness, used for detailed, quantitative measurements in materials science and engineering. As single-crystal aluminum oxide (Al₂O₃) sapphire has a Hardness Vickers of around 2000 to 2300 HV. Regular glass reaches around 500 to 700 HV and mineral glass approximately 900 HV. And 316L stainless steel for instance has an HV of about 150 to 200.

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While sapphire is durable, it can still be prone to shattering upon strong impact, which is its main weakness compared to other materials like acrylic or mineral glass, which are more flexible. Although not suitable for the case of a watch.

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Compare case materials on the Hardness Vickers scale (from hard to hardest):

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Watches from sapphire