Sapphire lenses can be used to protect watches. The sapphire used for watches is synthetic sapphire, which is a hard and transparent material made of high-temperature crystallized alumina that is very scratch resistant. Its hardness is higher than mineral glass or organic glass, but it is also more brittle.

From a chemical perspective, synthetic sapphire can be used for jewelry like natural sapphire, but does not require coloring agents to give the gemstone color. Artificial crystals take on a spherical shape and are then sawed into pieces using a diamond saw. These discs are then ground and polished into watch crystals. Whether natural or artificial, sapphire is a high hardness substance with a Mohs hardness of 9. Sapphire lenses are often marketed as "scratch resistant", which means they are difficult to scratch and damage, but some high hardness substances can still cause scratches on them. Sometimes these materials are used on furniture or walls, watch wearers should be aware that if the sapphire watch mirror accidentally scratches the surface of this material, it may cause scratches.

Some mineral glass crystals are also promoted as scratch resistant, with a hard coating on their surface that helps them resist scratching. One or both sides of the "anti reflective" crystal are coated with a substance (the same substance used on reflective glass) to reduce reflection and make the surface more readable. Anti reflective crystals can be made from mineral crystals or synthetic sapphire lenses, and an interesting feature of these crystals is that they do not reflect any light when viewed from the front because they do not reflect any light. In some cases, the coating can turn the crystal blue, like glass, making it easy to distinguish whether the watch has a light colored dial.