Crystal in this case is not the quartz crystal in the watch movement, it refers to the glass which protects the watch from dirt and water. There are three main types of crystals used in horology: sapphire, mineral and acrylic.
Sapphire crystal (or synthetic sapphire)
This material is a synthetic compound grown in laboratories. It has exactly the same composition with natural sapphire but much lower price.
Sapphire is the second hardest element after diamond so it is scratch and crack resistant, which can keep your watch look brand new despite constant use. Because of this, sapphire is commonly used in luxurious watches. However, it leads to the high cost in manufacturing sapphire crystals as this material needs special diamond tipped cutters to be shaped.
The major disadvantage of sapphire crystals is that they are brittle and easy to chip or shatter. If this happens, sapphire particles can get into the movement and cause devastating damages.
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Acrylic or pexiglass is similar to plastic. It is light, durable and the most affordable type of crystal but prone to scratches. That’s why it is most suitable for children watches but not for fancy timepieces. Acrylic crystals have a unique advantage: it can be polished to remove tiny scratches.
Mineral glass is a fancy word for ordinary glass which has been heated or chemically treated to increase scratch and shatter resistance. Mineral crystals are quite durable but less durable than acrylic crystals. They will get some scratches eventually and not as easy to be polished as acrylic crystals.
When it comes to replacement, this type of glass usually doesn’t cost you more than one hundred dollars.
Mineral crystals are broadly used on low-price & middle-class watches.
Notes in maintenance
Sapphire crystals are most recommended for their scratch resistance but it isn’t perfectly resistant to withstand any impact. You still have to take good care of your watch, such as wrapping it with cloth when not using. More importantly, avoid direct contact with other jewelries, with gem edges and other glass surface.
Sapphire is also believed to be easy to chip or shatter, so avoid physical abuse such as doing sport. It will cost you several hundred dollars for replacement.
When your crystal is cracked or heavily scratched, immediate replacement is needed because water or dust can penetrate into the movement, affects the performance or even damage your watch.
Last but not least, no matter what the crystal is made of, your maintenance is the most important factor in preserving a watch. Do not rely too much on the crystal material, treat your watch with care and it will remain in its best condition.