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The clear cover of a watch's face is usually referred to as the crystal - the crystal is the eye of the watch and through it one might see its soul! This element is designed to protect the dial and inner movement of the watch and needs to resist scratching and rupture.
The first portable watches like the ones manufactured by Peter Henlein at beginning of XVI-century were not provided of crystal. Those early watches were, instead, using a metal element or cage to protect the hands and the watch movement. The crystal has been introduced only at a later stage and sometimes still in combination with an external metal cage (e.g. Waltham WWI wristwatch).
Since then many different materials have been explored to protect the watch phase. Sapphire crystal has been introduced around 1930 but broadly adopted by luxury timepieces only over the last 30 years (driven by Rolex). The main reason for the slow adoption was the manufacturing complexity and the cost associated.
In this chapter we cover the different types of watch crystals as well as the manufacturing process of sapphire crystal.