Almost every watch has some type of glass that protects the hands and complications from damage. Most of us don't pay attention to its material, from a long-term point of view it is quite good to know the difference between different materials.
Cheap plastic? Isn't the worst thing
Some, mostly cheaper and childrens watches, have an organic or acrylic glass cover, i.e. plastic. Although it differs from ordinary plastics, which you know from everyday life, it is still available and above all easily scratchable material. Why would you what that in your watch?
Plastic covers are characterized by lower scratch resistance, but are more flexible and impact resistant. Moreover, when it breaks, the fragments are not sharp. Main reason why it's used in children's watches. Watches with plastic lids also fly into space, so mockery of this material is definitely not in place.
Mineral glass is the gold standard
We dare say that in most watches you will find mineral glass. They are characterized by good durability, higher flexibility compared to sapphire glass or higher scratch resistance compared to plastic glass. On the Mohs scale, mineral glass occupy grade 6, with grade 1 being talc and grade 10 diamond.
The low susceptibility to glare formation is also a valuable feature. Some companies improve the classic mineral glass with additional properties. For example, Seiko offers a watch Hardlex with a hardness grade of 7.
The sapphire glass is the king
Right at the beginning - the expression sapphire glass doesn't mean that it's made of sapphire, that is found in the wild. It is a chemical sapphire, a material that is, with its qualities, similar to a real sapphire. On the Mohs scale, it's at 9, with only the diamond being harder.
You can only scratch it with another material with a hardness of 9 or higher, which is unlikely in normal use. Non-scratchability, however, cannot be guaranteed, so it is not subject to complaints. Not scratching does not mean being unbreakable - if you manage to break a sapphire crystal, watch out for a lot of small and sharp pieces.
Which glass is the best?
There isn't a clear answered. Different situations require different glasses, so it depends primarily on the nature of the watch. Children and cosmonauts needs just plastic, with the suit reach out for sapphire watch. And for all other cases, a mineral glass will work jsut fine.