There are different types of watch glass that protect the dial of a wristwatch. One variant is the so-called mineral glass. A distinction is made between hardened and untreated glass. The chemical name of this watch glass is silicon oxide (SiO2). If other oxides such as boron oxide or aluminium oxide are added to the glass, it is refined and becomes harder and more scratch-resistant. Unlike plastic glass, mineral glass does not turn yellow. However, it cannot be polished as easily as plastic glass and is not resistant to damage. If the mineral glass is severely damaged, it can also break, whereby fine glass fragments can reach the movement and cause considerable damage. The mineral glass is hardened in a chemical process. This makes it more resistant and more shock-resistant, so that the probability of shattering is reduced.
