Book 27, Question 111
Why are watch faces and camera lenses made of sapphire?
Lab-grown sapphire can be made into clear windows that resist scratches from keys, sand, and dust.
Sapphire is a form of corundum, a crystal that ranks 9 on the Mohs scale, harder than almost anything except diamond. It is used on fine watches, phone camera covers, and supermarket scanner windows. Because it's made in a factory, it doesn't have to be mined.