A watch crystal often called a watch glass is a transparent lens which protects the watch face. Crystals can be made out of four materials as below:
- Perspex or Plexiglass (this is a clear plastic material)
- Conventional glass which is similar to that used for windows
- Synthetic sapphire
- Sandwich - Sandwich is a combination of mineral and sapphire glass. The usual method is to cover mineral glass with a layer of synthetic sapphire. Seiko used variations of this material called "Sapphlex and Hardlex” . These material were supposed to give the scratch resistance of Sapphire with less brittleness but I have not seen them recently.
The advantages and disadvantages each option
Plexiglass or Perspex, is the least expensive. It is popular with some military manufacturers such as CWC because it is the least likely to shatter but on the negative side with is very easily scratched although toothpaste will remove minor scratches and there is a specialist products called Polywatch which is even better and a video appears below.
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We find that with Mineral glass, even though it has often been hardened by a tempering process, is somewhat more likely to break than plexiglass. On the plus side it is significantly more scratch resistant than Plexiglass. Of the main options Synthetic sapphire is by far the most expensive option and also the most scratch resistant on the negative side it is also quite brittle, and shatters more easily than mineral glass or plexiglass and in military applications the fact it breaks into small pieces can pose significant issues.
Synthetic Sapphire or even natural Sapphire is one of the hardest substances know. It measures 9 on the Mohs scale, Mohs is a system for rating the relative hardness of various materials. (the highest rating is a Diamond which measures 10) Although you will often find that Sapphire watch crystals are marketed as "scratch resistant", it is not impossible to scratch them. We have found they can be damaged by contact with a Diamond, for example a ring can scratch them as can anything containing silicon carbide which is rated lower than a Diamond on the Mohs scale but is a bit harder than Sapphire. In terms of appearance a Plexiglass crystal will look very different to glass and a tap with a fingernail will identify it as plastic by sound. With Mineral and Sapphire glass they are hard to tell apart and a scratch test is the only way to tell which is impractical in most cases. As a point of interest should you be offered a box of crystals as spares a pocket knife of screwdriver will generally mark a mineral crystal where it won’t mark a Sapphire crystal.
Most manufacturers such as Rolex, Breitling or Tag Heuer will often use Sapphire crystals but military manufacturers tend to use Plexiglass / Perspex or hardened Mineral Glass this is because as explained earlier when a Sapphire watch glass shatters small shards of glass can go everywhere and may lodge themselves in the movement. If this happens to a mechanical watch then the cost of the repair may well exceed the value of the watch. If the glass does shatter keep the watch fact down until you get it to a watchmaker in order to prevent this from happening.