Sapphire crystal is more reflective than mineral crystal due to its higher index of refraction. (1,8 compared to 1,47). Applying one or more layers of AR (anti-reflective) coating will limit this reflection to a very low level. Very often, lines and marks on the top surface of a sapphire crystal are mistaken for scratches but are actually the scratched AR coating, or just AR coating wearing off after time. An AR coating on the inside of the crystal provides a good AR performance while avoiding this problem.