Crystal is the clear part of a watch case that covers the dial. It is also called the window or the watch glass. It guards the dial and hands, and keeps dust, moisture and knocks away from the dial. Common materials include acrylic, mineral glass and synthetic sapphire.
What is an acrylic crystal?
Acrylic is a plastic. It was developed in 1928, and the Rohm and Haas Company put it on the market about five years later. It is sold under several trade names, including Perspex, Plexiglas, Acrylite, Lucite and Hesalite. Acrylic is the cheapest crystal material, which is why nearly every low-cost watch uses it. Its weakness is scratching. It rates only about 2 to 3 on the Mohs hardness scale, so it marks very easily. The best-known modern watch with an acrylic crystal is the Omega Speedmaster Moonwatch. Its crystal is called Hesalite.
How do mineral glass and sapphire crystals differ?
Mineral glass is usually treated with heat and chemicals. This makes it harder and less likely to shatter. Synthetic sapphire resists scratches better than any other crystal material. Sapphire is the gemstone name for corundum, which is aluminium oxide. It rates 9 on the Mohs scale, just below diamond at 10. That hardness has a cost: sapphire is hard to cut and polish. Many crystals also carry an anti-reflective coating. These coatings have become more common, and they are usually applied to the underside of the crystal.
Where it comes from
The earliest clocks had no protective cover at all over their dials and hands. Early pocket watches had a clear cover made of rock crystal. Rock crystal is a natural kind of quartz. The part takes its name from that material. Today the same name is used for clear covers of acrylic, mineral glass and synthetic sapphire alike.
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