An anti-reflective coating is a coating that reduces the reflections on a sapphire crystal when it is exposed to light. Light consists of different wavelengths, and an anti-reflective coating can only absorb some of the wavelengths. This is often visible, in that the sapphire crystal has a bluish or greenish residual reflection.
An anti-reflective coating is not as scratch-resistant as a sapphire crystal. For this reason, damage to the anti-reflective coating on the outside of the sapphire crystal can be perceived as a scratch, as the anti-reflective coating is missing at this point and this area stands out strongly. For this reason, the majority of sapphire lenses only have an anti-reflective coating on the inside.