The term refers to the transparent covering over the face of a watch. Another term used is watch crystal. They can be made from different kinds of material but it all depends on the watchmaker, the uses and the quality.
There are three materials from which the crystal can be made of: acrylic glass, mineral glass and sapphire glass. The acrylic glass is actually plastic and it is the cheapest. The plastic is resistant, transparent and also flexible. It cannot break because it is rather light but the downside is that it can scratch very easily. These scratches can be removed but the job should be done by a professional. The vast majority of acrylic watches are meant for the children because it is a known fact that kids are not always careful.
The other type of glass is the material glass and this is the most commonly used. This mineral glass is made out of ordinary glass through heat and chemicals. This glass can also scratch easily but the good thing is that it can be replaced easily. If the glass breaks, then the small pieces can enter the interior mechanism and affect it. If this happens then you might consider buying another watch because the repairs can cost more than the watch itself.
The third type of crystal is the sapphire crystal. This is a very hard and transparent crystal made out crystallizing aluminum oxide at very high temperatures. Also, one can buy a watch that has a genuine sapphire glass but these are usually more expensive. This type of glass is the most durable and it is the most unlikely to shatter or to scratch. The high price of a watch with a sapphire glass is due to the precious stone, but also because it is shaped with expensive tools that use diamonds. The most famous brands often use this type of glass for their watches.
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