A watch crystal is a transparent cover that protects the watch face. Note that, coincidently, the word “crystal” is also used to denote the tiny piece of quartz that serves as an oscillator in a quartz watch. These two types of crystals have nothing to do with each other. The latter is usually called a “Quartz Crystal” to prevent confusion.
They can be made of any of three materials:
- Plexiglass (a clear, lightweight type of plastic)
- ordinary glass as used in windows, and usually referred to in the watch business a “Mineral Glass”
- synthetic Sapphire
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 call this composite material “Sapphlex.”
Plexiglas, 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 a tempering process has hardened it, is more likely to break than a Plexiglass. But it is also more scratch-resistant. 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 or Plexiglas.
It is a very hard, transparent material made by 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. (Diamonds measure 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. Diamonds can scratch them; so can man-made materials that incorporate silicon carbide, 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.
No. Mineral glass and sapphire generally look the same. A sure way to tell them apart is with a scratch test. “You would not be very popular if you have to take a new watch from a showcase and do a scratch test on it.” A steel knife or screwdriver will scratch a mineral glass but not a sapphire.
No. Synthetic sapphire was invented in the 19th century and first used for watch crystals in the 1960s. Now really all high-end watch brands use synthetic crystals in at least some or their models.
No. Some mineral-glass crystals are also marketed as “scratch resistant.” These crystals have a hard coating that makes them less likely to get scratched.
All are French words that refer to the shape of the crystal. “Lunette” simply means round-like a full moon (lune means “moon” in French). Bombé, Chevé, and Boule all means concave, or dome-shaped. There are other words used to describe watch-crystals shapes. A “raised” crystal is flat on top but dome in the center. “Shaped crystals” are any that are not circular. Rectangles, square and ovals being the most common.
This type of crystal has been coated on one side or both sides with a substance that makes it easier to read the dial of the watch in direct sunlight. One interesting feature of these crystals is that is has a distinctive tint to it. At they use it often in most models.
Consumers can expect to pay anything from R80 to R160 for a plexi glass to more that R600 for Synthetic sapphire one. (At Omega, Rolex and other top-end brands, for example, synthetic sapphire crystals range from R600-R1200. In general, the more expensive the watch the more you will pay for a crystal replacement.