Sapphire is a general term for corundum gemstones of colors other than ruby, and many watches are now made of sapphire material. According to chemical principles, its main component is aluminum oxide (Al2O3), and the blue color is caused by a small amount of titanium and iron impurities. Sapphire lenses have high hardness, and watches made from them have higher impact resistance, airtightness, and strength than other materials, which is also an important manifestation of watch quality.

Sapphire lenses are artificially compressed crystal mirrors, which have the advantages of high density and are not easily scratched. Whether it is naturally formed sapphire or artificially synthesized sapphire, it is one of the harder substances, comparable to diamond, with excellent quality. Its hardness is 2200-2300 Vickers hardness (7 times that of steel hardness) and 9 Mohs hardness. Good hardness, good transparency, low friction, high temperature resistance, and higher wear resistance than crystal glass mirrors and synthetic glass mirrors. Mohs hardness, also known as Mohs hardness, is a standard for mineral hardness. Proposed by German mineralogist Frederick Moss in 1812, the measured scratch depth is divided into ten levels to represent hardness: talc level 1, gypsum level 2, calcite level 3, fluorite level 4, apatite level 5, orthoclase level 6, quartz level 7, yellow jade level 8, corundum level 9, and diamond level 10.

But it is not indestructible either. Some silicon carbide, stones, and high hardness substances can also scratch sapphire watch lenses, so when purchasing a watch, it is necessary to check whether the watch itself has been scratched. During the delivery process, some external and human factors such as collisions, falls, or improper operation can cause damage to the watch mirror. Even if sapphire lenses are hard and do not cause significant damage, the presence of scratches can still reduce their aesthetic appeal.