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What is a watch crystal?
Simply put, they are the glass or plastic component of the watch. They function as a window to protect the hands of the watch, but still allow it to be visible. The crystal also protects the movement of the watch itself from the environment to varying extents. This can be anything from general protection to being waterproof under deep sea diving conditions.
They can be made of several different materials which include glass, acrylic or a material known as sapphire glass. The biggest concern with a crystal is breakability, which contracts the options that are available that you can use to manufacture a watch. They need to be tough to handle the day to day bumps and hits that watches absorb.
Acrylic
By far the cheapest option, acrylic or plexiglass watch crystals are tough and durable, they’re very hard to break, and they are transparent and low-glare, but the one thing they aren’t is scratch-resistant. Under normal day to day use, they will slowly accumulate scratches until they become increasingly opaque until finally the watch face disappears completely. If you swipe one against a sharp object, it’s a disaster.
The good news is that that doesn’t damn a watch with an acrylic crystal to the trash can when they get scratched, they can be easily buffed back to transparency, with varying degrees of success, but in general the use of acrylic crystals by a company tends to indicate that the watch is a cheap, mass market watch.
Glass
Often called “mineral glass” the fact is it is generally just a glorified tempered formulation of regular glass. It’s much harder to scratch than acrylic, and covers most mid-priced watches, but it’s also much easier to shatter than acrylic. The biggest problem with tempered glass is glare.
Sapphire Crystal
Sapphire glass is considered the cadillac of watch crystal materials. There is a good reason for this, sapphire glass is highly transparent, but also extraordinarily scratch-resistant and provides a compromise between both worlds when compared to glass or plastic. This is hard stuff with a value of 9 on the Mohs scale, so much so that it will set off many thermal diamond testers and in fact is the third hardest natural substance on earth after diamonds and moissanite.
Made at very high temperatures, aluminum oxide is turned into synthetic sapphire, but without the natural impurities that color natural sapphires making it perfectly clear. Sapphire crystal has a very wide optical transmission band ranging from ultra-violet to near-infrared covering the range of human vision and an extremely high melting temperature, nearly 2030 °C. Sapphire crystal is often called “sapphire glass” but in fact it has very little to do with glass. In reality, it’s actual crystalline mineral sapphire. The synthetic sapphire is grown in specific orientation to admit light. More on this process later, but suffice it to say that the crystals are grown in such a way as to transmit light as transparently as possible, even more so than the glasses used in many astronomical telescopes.
After being grown, round masses of synthetic sapphire crystals are sliced and then highly polished to achieve the desired surface finish with diamond-coated saws and equipment. There are a wide range of finishes, though generally transparency is still key as a rule. One of the things that contribute to the cost of a sapphire watch crystal are the materials needed in polishing such a hard material.
But there’s a problem. A sapphire crystal, because of its hardness, is more easily shattered than both acrylic or tempered glass. And for that reason, we arrive at a subgroup, the hybrid.
Hybrids
This type of crystal, favored by companies like Seiko, employ the shatter-resistant properties of tempered glass with the scratch-resistant properties of sapphire by coating the tempered glass with sapphire. Seiko terms this “Sapphlex” and sits at the top of the heap as far as functionality.