A watch's focal point is its dial — you don't want to worry about the glass you've got to peer through. And yet, the pros and cons of different varieties of watch crystals are endlessly debated by enthusiasts. More than just a practical concern, a good crystal can significantly impact the design, user experience and even of a watch in several ways.
You'd be forgiven for being confused by some of the technical details found in watches' product descriptions regarding crystals. Is sapphire better because it's more expensive than acrylic? What the hell is a "box" crystal, anyway? And should you care about any of that? If you're buying a watch, the answer to the latter question is certainly a "yes." The various materials, shapes and other aspects of watch crystals, however, require a bit more breakdown:
What Do You Need to Know About Watch Crystal Materials?
The most common types of watch crystal materials are acrylic, mineral and sapphire, and each has its benefits and tradeoffs. While their physical properties are undisputed, preferences can vary between sapphire and acrylic in particular.
Acrylic: Acrylic is a kind of plastic, also called Plexiglass, Hesalite and other names, and it's the crystal material you'll find on the majority of vintage watches. It scratches easily, but tends to crack rather than shatter, and this has led to some instances where it's been preferred over sapphire: the logic is that a watch can remain functional with a cracked crystal but would be totally destroyed if the crystal were shattered. This is a reason you'll find it used on some watches meant for rugged use, such as dive watches or, notably, the Omega Speedmaster Moonwatch (Hesalite version).
Some collectors also prefer the look of acrylic for aesthetic reasons, citing its "luster" or "warmth" and the particular way it distorts around the edges. The other benefit of acrylic is that it's inexpensive and can be easily shaped. While it indeed scratches easily, light scratches can be easily buffed out at home.
Mineral: Mineral crystal is formed from glass and hardening minerals, and it's most common among the affordable end of watches, particularly those from the big Japanese companies. It'll chip rather than shatter, and while mineral is more scratch-resistant than acrylic, it does scratch and the scratches can't be easily buffed out. Seiko's own version of mineral crystal called Hardlex is further hardened to improve scratch-resistance.
Sapphire: The overwhelming choice for luxury and high-end watches, sapphire as used in watch crystals is a synthetic material (as opposed to the natural gemstone). One of the hardest materials on earth, it's virtually impossible to scratch — an illustration of its scratch-resistance are the many flawless crystals which can be found on watches whose cases have been visibly beat up over the years. It also has impressive clarity, which can make the dial elements appear sharper and generally results in a clean, high-end feel.
When untreated with anti-reflective coating, however (more on that below), sapphire crystal can also be highly reflective, which is detrimental to legibility. Also, while other materials might crack or chip, sapphire can shatter — though it takes an extremely hard impact and is an uncommon occurrence.
The primary drawback of sapphire crystal is that it tends to be more expensive. It requires high-tech production, and its hardness makes it difficult to work with, especially in complicated shapes. With improved technology and growing sourcing options, however, more entry-level brands have begun to offer it and high-end brands have been able to do more with it, such as producing entire watch cases in sapphire.
What Are the Pros and Cons of Different Watch Crystal Shapes?
It's not just the materials you want to consider: a crystal's shape can make a big difference both for legibility as well as overall watch design and aesthetics. Domed and flat are the most common types you will find, but not all are created equal. Some models are more domed than others, and flat crystals can also feature a raised edge that makes them more visually interesting.
One of the most concrete examples of how a crystals shape can affect your experience is on dive watches: a domed crystal can make a dial go completely blank underwater when viewed at an angle, whereas flat crystals remain more legible. Flat crystals might offer better legibility in some situations, but they don't offer the depth and elegance of curved ones.
When evaluating a watch, you should always look for the ability to read the time easily from any angle in any situation. When a watch features, for example, a domed outer surface and flat inner surface, extreme visual distortions can occur. That's why watch brands proudly advertise "double-domed" crystals, meaning both outer and inner surfaces are curved and will offer a clear view of the dial, even from angles.
An interesting type of crystal is what's referred to as "box-shaped." These are crystals which are prominently raised a couple millimeters above the bezel so its edges are easily visible. This is common among vintage watches that use acrylic crystals, but advancing technology has allowed more watchmakers to offer the shape executed in sapphire — negating the obvious problem that the protruding shape is an absolute scratch-magnet for acrylic crystals. A box crystal can add visual interest with its three-dimensional look as well as give modern watches a more vintage feel.
Watch Out for Reflections
Like every aspect of watches, you will find forum nerds debating the merits of something called anti-reflective (or "AR") coating. Also used for things like eyeglasses, telescopes and other lenses, AR coating is most important in watches for glare-prone sapphire crystal. These coatings can reduce reflections as well as improve contrast, so what could possibly be wrong with that?
Coatings on sapphire crystals are mostly not very noticeable, but can become more apparent when scratched. There are different types of AR coatings used by different companies, and some are more easily scratched than others. To avoid unsightly scratches, some watchmakers only coat the underside of the crystal (which if left uncoated can reflect dial elements), but this is a compromise and can result in a noticeably reflective crystal. Some enthusiast also complain about the tint sometimes visible around the crystal's edges or in reflections.
Whatever its materials, shape or other factors, a well made and nicely shaped crystal can be a pleasure in and of itself and elevate a watch. If all the above seems like a lot to consider when watch shopping, at the very least keep in mind to evaluate a crystal's legibility when watch shopping.