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Watch Talk: Watch Crystals

1.What is a watch crystal?
A watch crystal is a transparent cover that protects the watch face.

2. What are watch crystals made of?
They can be made of any of three materials: 1- plexiglass (a clear, lightweight type of plastic), 2- ordinary glass – like that used for windows, and usually referred to in the watch business as “mineral glass” or 3- synthetic sapphire (see question 4). Some crystals are made of both mineral and sapphire glass. Seiko, for example, makes some watches with crystals made of mineral glass covered with a layer of synthetic sapphire. Seiko calls this composite material “Sapphlex”.

3. What are the advantages and disadvantages of each material?
Plexiglass, as you would expect, is the least expensive. It is also the least likely to shatter and the most likely to become scratched. Mineral glass, even though it has been hardened by a tempering process, is more likely to break than plexiglass. But it is also more scratch-resistant than that material. Synthetic sapphire is the most expensive glass crystal material and the most scratch resistant. Because it is so hard, it is also brittle, and shatters more easily than mineral glass or plexiglass.

4. What exactly is synthetic sapphire?
It is a very hard, transparent material made of crystallizing aluminum oxide at very high temperatures. Chemically, synthetic sapphire is the same as the natural sapphire used in jewelry, but without the coloring agents that give the gemstone its various hues. When it is heated, the synthetic sapphire forms round masses that are sliced into pieces with diamond-coated saws. These disks are then ground and polished into watch crystals. (One reason sapphire crystals are relatively expensive is that the tools required to make them are costly.) Sapphire (whether natural or synthetic) is one of the hardest substances on earth. It measures 9 on the Mohs scale, which is a system for rating the relative hardness of various materials. (Diamond measures 10, the highest rating.) Watch crystals made of synthetic sapphire are often marketed as “scratch resistant”, meaning they are very difficult – but not impossible – to scratch. Diamond can scratch them; so can man-made materials that incorporate silicon carbide, which, with a Mohs rating of between 9 and 10, is, like diamond, harder than sapphire. These materials are sometimes used to make simulated-stone surfaces for furniture or walls. The watch wearer should note that accidentally scraping a sapphire crystal against such a surface could cause a scratch.

5. Can you tell if a crystal is made of sapphire by looking at it?

Not usually.. Mineral glass and sapphire generally look the same. A surefire way to tell them apart (albeit an often impractical one) is with a scratch test. A stainless steel knife or screwdriver will scratch a mineral-glass crystal but not a sapphire one.

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