Watch Crystals - Windows to Time

Go back five centuries and the original “pocket watches” were more like miniature clocks worn around the neck. There was a single hour hand that was fully exposed, not protected by a crystal as virtually all watches are today. Decorated lids kept them safe, but this was no longer adequate as portable time became mainstream in the 17th century. The newly invented waist coat was also designed to carry them in a dedicated pocket. It was the dawn of modern pocket watches and glass crystals became mandatory. Fast forward to today and there’s several crystal materials, from sapphire to mineral glass to acrylic. It’s debatable as to which is best, but sapphire is generally accepted as superior.

Sapphire

Sapphire crystals are the norm for high end watches and they’ve steadily trickled down to mid-range pieces as well. Even inexpensive watches, well south of $1,000, have sapphire crystals today as production has become easier. Introduced in the 1960s, sapphire crystals look and feel like glass, but they’re actually the crystallization of pure aluminum oxide under extreme heat. The result is one of the hardest and most scratch resistant materials, measuring 9 on the Mohs scale and second only to diamonds (10 on the Mohs scale). It has the same hardness as natural sapphire (hence the name), but with perfect colorless transparency. Although very scratch resistant, they’re not 100% scratch proof and hard knocks and rough wear can result in imperfections over time. Our entire Horage portfolio utilizes sapphire crystals as they remain the highest quality and most resilient in the industry.

Sapphire Disadvantages

An issue with sapphire is its stubbornness with reflections, so anti-reflective (AR) coatings are generally necessary, especially with darker dials. These coatings are very effective and can be applied on both the inside and outside of crystals, although outside coatings can be scuffed and damaged (not hurting the crystal itself), while interior coatings are completely protected. Almost all high-end and most mid-range watches have AR sapphire crystals, while lower end pieces often skip this step. The benefits of uncoated sapphire in inexpensive watches usually outweighs this disadvantage.

A second issue is that sapphire can’t be repaired, so even light scratches are permanent and the crystal must be replaced for a flawless surface. Mineral glass has the same issue and only softer acrylic crystals can be polished to remove scratches and imperfections. Given the hardness and resiliency of sapphire, this is usually not a problem and sapphire crystals can remain virtually flawless over a lifetime.

Interestingly, sapphire is sometimes used for more than just crystals. Its hardness and durability allow it to be formed into case band windows or even entire watch cases, although this is usually reserved for very expensive pieces. Sapphire cases provide unique and unfettered views of the movement from multiple angles, allowing watchmakers to really showcase their craftsmanship.

Mineral Glass

Mineral glass is a less expensive option and provides a hard, durable surface for accessibly priced watches. It’s much easier to manufacture, requiring more common tools and methods, and is also easy and inexpensive to replace. It’s not uncommon for a mid-range watch to have a sapphire front crystal and mineral glass display back as the latter is naturally protected by the wrist and less likely to be scratched. Mineral glass crystals can be tempered, meaning they’re heat treated to improve hardness and scratch resistance. This is often called K1 glass, although some brands have a special name - Seiko calls it Hardlex, for example. Like sapphire, scratches in mineral glass can’t be repaired and the crystal must be replaced. Mineral glass is naturally less reflective than sapphire and it’s less common for them to have AR coatings, reducing the cost even more.

Although tempered mineral glass is durable and scratch resistant, it’s not nearly as hard as sapphire and will likely show some wear over time. Hardness falls around 5 on the Mohs scale. This would be unacceptable for an expensive watch these days, but it's a solid option for something more affordable. With a bit of care, the crystal should remain trouble free.  

Acrylic

Acrylic is a fancy way to say plexiglass (aka plastic). It’s specifically Polymethyl methacrylate, which is a durable thermoplastic that doesn’t shatter like glass. Some brands have their own spin on this - Omega calls their acrylic crystals Hesalite. At first glance, an acrylic crystal seems very inferior to glass or sapphire, but there’s more to it than overall durability. There’s a long history of acrylic crystals in the industry with major brands like Rolex and Omega embracing it for decades. In fact, Rolex used acrylic for much of its life, only beginning to transition to sapphire in 1978. It wasn’t until the early 1990s that the brand stopped using acrylic altogether. Omega’s “Moonwatch” famously had an acrylic crystal and some current models retain it for an authentic feel. NASA couldn’t risk a glass crystal shattering and shards floating within the space capsule.

Acrylic was introduced to the watch industry in the early 1930s and became the standard for watches in World War II as it generally wouldn’t shatter.

Advantages and Disadvantages

Acrylic does have its advantages. For starters, scratches can often be buffed out, unlike glass or sapphire that can’t be repaired. As mentioned, it’s also shatterproof in comparison, which is important if there’s a high risk of impact for certain jobs, etc. There’s a warmth to acrylic that many collectors appreciate and it often provides distortions at angles that have that vintage vibe. Sapphire crystals can be shaped similarly, however, to mimic those distortions without the relative fragility. Acrylic has the least clarity of the three crystals, but that optical disparity again creates a warmth that’s all the rage for both collectors and new, vintage-inspired heritage pieces. Look no further than a modern Moonwatch.

The main disadvantage of acrylic is its vulnerability to scratches and scuffs. Although many can be buffed out by hand, deeper scratches can’t and a crystal can only be polished so much before its integrity is compromised. For most enthusiasts today, acrylic isn’t worth the hassle and guaranteed scratches that will form over time. Although once among the most common crystal types, it’s now a rather niche material for higher end pieces. It does remain a staple of cheap watches from brands like Swatch, Timex and other department store favorites for those that just need a cool and stylish timekeeper that won’t break the bank.

We Want to Hear From You!

Sound off in the comments about additional topics of interest, so we can be sure to cover what you want to read in future articles. And please share this with friends and anyone interested in watchmaking, and let's keep the conversation going. Also, be sure to sign up for our newsletter (here) and keep an eye out for new products as we're always in the process of development. Check out journal entries as well (here) and a detailed section on performance watchmaking at Horage (here).

We'll talk again soon!

Erik Slaven