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A Sapphire is a precious stone.

This gem of hardness 9 (after Mohs) comes in shades of blue, green and yellow or colorless. The colorless variant, very rarely also the blue, was sometimes used as bearing stone (jewel) and capstone in high quality pocket and wristwatches.

With high quality watches, it is now generally common that the watch glasses are made of synthetic sapphire crystal.