Sapphire, with a Mohs rating of 9.0, right behind to diamond. Three-layer coating process technique (Anti-Fingerprint Coating, Anti-Reflection Coating, High Transmittance Coating) Remarkable Anti-pollution ability, High Transparency. Clearly readable even under the sun bright. Highly resistance to heat, corrosion and with Ultra-Cleanliness. Not only much more solid and more durable with superb-comfort, but also harmless to skin in use.