Mineral crystal, sapphire crystal, sapphire coated mineral glass, acrylic glass… The glass of the watch affects the durability of your watch, so you should pay attention to it. How do the the watch glasses differ?
Different types of glass
The watch glasses are made of three different materials. Acrylic glass, mineral glass and sapphire crystal. There are also different coatings and variations of these materials, which we will now delve into.
Acrylic glass or plexiglass
Acrylic glass is a material made of plastic and is sometimes used as watch glass. As a soft material, it gets easily scratched. However, it can be easily polished a few times. As a flexible material it is quite impact resistant. As a material, it is inexpensive and it can be used for example in children’s watches.
Mineral glass is a material made out of glass. Almost all mineral glass watches today are tempered mineral glass. This means that the surface of the glass is heat-hardened, i.e. heated to a high temperature, which increases the hardness of the surface of the glass and thus does not break or scratch as easily.
The mineral glass is relatively hard and durable but does not give up at all in the event of an impact but shatters. If scratched, the mineral glass cannot be polished and must be replaced. However, it withstands relatively well in normal use and requires a hard impact to break.
Sapphire coated mineral crystal glass
To compensate for the properties of mineral glass, many manufacturers use mineral glass with a thin layer of sapphire. It has better impact resistance than mineral glass and in practice sapphire crystal can only be scratched with diamond. Thus, a thin layer of sapphire on top of the mineral crystal glass greatly increases its impact resistance and is not easily scratched.
Sapphire crystal is nowadays made of synthetic sapphire. Synthetic sapphire glass is not really glass at all. It is a very hard, transparent material made by crystallizing pure aluminium oxide at a very high temperature. Synthetic sapphire has the same hardness as natural sapphire.
When heated, it forms a pulp which is then cut into pieces with diamond coated saws. These plates are cut and polished into watch glasses. One reason for the price of sapphire crystal is the tools needed to cut and polish this very hard material.
Sapphire (either natural or synthetic) is one of the hardest substances. It has a hardness of 9 on the Mohs scale, which is a system for estimating the relative hardness of different materials. (Diamond being 10, the highest rating.) Watch glasses made of synthetic sapphire are often marketed scratch-resistant because they are very difficult to scratch due to their hardness.
At Aarni, we use both sapphire coated mineral glass and sapphire crystal glass in our watches.
Information about the materials of the watch can be found on product pages.