It’s actually a contradiction: The most important part of a watch, the movement, is just under the dial and a layer of crystal. Due to its exposed position, watch crystal in particular comes into direct contact with all the elements that watch wearers are regularly exposed to. In this article, you can read everything you need to know abut the crystal that so reliably protects the insides of our wristwatches.
The History of Watch Crystal
In the past, when pocket watches were still predominant, a spring cover served as protection for the watch crystal and dial. These pocket watches with covers were also known as savonette watches (French for “little soap”). However, with the advent of wristwatches, both pocket watches and their spring covers gradually disappeared from everyday life. The watch was now on the wrist, where it could be read much more comfortably, but was no longer as well protected. As wristwatches always had the function of a tool in those days, they naturally had to be adapted to the circumstances of their wearers. In addition to the everyday conditions watches were exposed to, watch manufacturers were also confronted with the difficulty that robust watch crystal was often less readable. That is why research has been and continues to be carried out to develop watch crystal that is both robust and easy to read.
What types of watch crystal are there?
We present the most important and most common types of watch crystal below. The types of watch crystal can be broken into three categories: mineral glass, plastic crystal, and sapphire crystal.
From Mineral to Acrylic Glass
The first watch crystal was made of untempered mineral glass, like that used in windows. However, this did not make the watches very shatterproof, and there was always a risk that the splinters could damage the dial or even the movement when the glass broke. At the beginning of the 1930s, plastic crystal, also known as Hesalite crystal or acrylic glass, came onto the market. Acrylic glass is made from polymethyl methacrylate (PMMA). This alternative to mineral glass was considered an innovation and was accordingly exclusive at the time. For example, Panerai used acrylic glass for its Radiomir watch models, which were supplied to the Italian Navy. Increased break resistance and good readability under water were of particular importance to the divers. However, as PMMA is slightly osmotic, even the smallest amounts of water can penetrate through the watch glass into the watch. In diving watches, this was counteracted by reinforced crystal. That means that the watch crystal is fixed in place by a metal ring, which also prevents the glass from shrinking or deforming under pressure and heat. Due to its high impact resistance, Omega relied on Hesalite crystal for their infamous Speedmaster, which was the first watch to fly to the moon.
However, plastic glass can scratch easily due to its low density. This is not necessarily a disadvantage, as it can be easily polished, but some might see it as a minor flaw. The dials of luxury watches with acrylic glass can also yellow over time and under prolonged UV exposure. For some watch enthusiasts, however, this can also be a reason to purchase a particular luxury watch. Especially with vintage Rolex models, this effect leads to a lot of buying interest.
Hardened Mineral Glass
Whereas mineral glass used to be thermally hardened, since the 1970s it has been hardened in a salt bath. That made it easier to adjust the stress distribution and even watch crystal just 1 mm thick became extremely resistant. Mineral glass can also be further refined by adding aluminum oxide and boron oxide, significantly improving its scratch resistance compared to plastic glass. Some manufacturers, such as Seiko, call that Hardlex glass. However, once scratches have occurred, they are not as easy to polish as plastic crystal.
Hardened mineral glass reaches a Vickers value of 400, which describes the hardness of materials. In this hardness test, the visual impression is measured after a load with a weight. As this test is easier to carry out than other hardness tests, the Vickers value has become the generally recognized value for hardness. Tempered mineral glass gradually replaced plastic crystal due to their greater resistance to breakage and their higher quality appearance. Only alarm clocks continue to use plastic watch crystals, regardless of their price segment.
Sapphire Crystal
The longer a watch with mineral glass is worn, the more you can see and recognize this. Over time, dials under heavily scratched watch glass can become almost impossible to read because their deep grooves cannot be polished. If you want to sell your used luxury timepiece, this is a particular hindrance. This was remedied by the synthetic production of sapphires using the Verneuil process. The process creates pear-shaped crystals, which are then cut, polished, and adjusted to the appropriate size for watch crystal. Sapphire crystal has a crystalline rather than an amorphous structure. Because the production process is complex and the raw materials required for sapphire crystal are expensive, watch crystal made from this material is found in luxury watches in the upper price segment, where it has now become standard. Only diamonds can damage the surface. Nevertheless, watches with sapphire crystal are often sent for refurbishment due to scratches. However, these marks are merely the residue of softer materials and can often be easily removed with an eraser. The only downside is the stronger refraction of sapphire crystal, which can cause annoying reflections. However, that is often counteracted with special coatings.
Comparison of Different Types of Watch Crystal
| Sapphire crystal | Mineral glass | Acrylic glass | |
| Scratch susceptibility | Very little | Somewhat higher | Much higher |
| Reflection | Very high(special coatings can help) | Little | Very little |
| Care/polish | Impurities on the surface can be removed with an eraser | No polishing possible | Polishing possible |
| Cost | Expensive | Affordable | Very affordable |
Testing a Used Watch for Crystal Quality
If you have purchased a second-hand watch or even a vintage model, it may sometimes not be clear how high qualty the crystal is. However, if you tap the crystal gently with your fingernail, you can clearly hear the difference between the various types of watch crystal. The sound of high-quality watch crystal is a muffled, deep tone. Lower quality crystal and models rattle and clink when you knock on the glass. In addition to an auditory test, you can also perfrom a visual test. All you have to do is place a drop of water on the watch glass and watch it run down the glass. If it slides down slowly, it is high-quality sapphire crystal. If, on the other hand, the drop runs down quickly, then it’s mostl likely a cheaper mineral glass.
Is crystal decisive when buying a watch?
As important as watch crystal is for protecting the dial, it should not be a decisive factor when purchasing a watch. Impacts that cause hardened mineral glass to break are generally not good for the inside of the watch either. Watch crystal alone does not make your watch more robust, which is why you should pay attention to the all-round quality of the watch. Ultimately, you should choose the watch that you like best. A very simple hint. However, given the large number of factors that lead to the decision for a watch, it is important to always be clear about what you like or dislike about watch models. In the end, you are guaranteed to choose the right watch for you. People have many different tastes, which in turn influences which timepiece from which brand is right. A model with acrylic glass, for example, can have a special appeal to watch collectors due to its patina. That’s because for many, the appeal of collecting watches lies not in the materiality of a watch but in the story it tells. On the other hand, an unrestricted, clear view of the dial through the watch crystal is what we most often expect from our watch.
