Glass made of artificially produced sapphire crystal (Al2O3 = monocrystalline aluminum oxide). With a hardness of 9 on the Mohs scale it has a considerably higher scratch resistance and a resistance to fracture than mineral glass. With these properties, it is only by exceeded the diamond (Mohs hardness of 10).
Today sapphire crystal is the material of choice as watch glass of high quality watches. In most cases, these glasses are anti-glare treated. There is unilateral or bilateral anti-reflective coating.
There are flat, arched and domed sapphire crystals. The difference in production methods is sometimes considerable and is reflected in correspondingly different costs
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