Watch crystals made of synthetic sapphire are often marketed as “scratch resistant” or “virtually scratchproof” because they are very difficult - but not impossible - to scratch. Diamond can scratch them; so can man-made materials that incorporate silicon carbide, which, with a Mohs rating of between 9 and 10, is harder than sapphire.

Optical crystals are used as view ports to protect other components within an optical assembly, instrument, or laser. In certain applications, optical crystals are required to withstand harsh environmental conditions and provide high levels of performance and reliability. Sapphire crystal has several unique properties that make it the ideal material for these applications.