In-Depth: Crystal Clear: More Than You Ever Wanted To Know About The Watch Crystal
There’s a saying in sports that “a good referee is one you don’t notice,” and sometimes it feels like the same could be said for the watch crystal. But there’s so much more to the component that we look straight through than meets the eye (or doesn’t).
Over the last century, a few materials have been used to cover the dial, mostly acrylic (or plastic), mineral, or sapphire crystals. Today, sapphire is most commonly used on watches of a certain price point, though appreciation for old-school acrylic continues unabated in corners of the collecting world. It’s time the crystal got its shine. Here’s more than you probably wanted to know about the history of the crystal, how it’s produced, and why some collectors prefer one over the other. I even spoke with a couple of crystal innovators to get a clear vision of the future of acrylic and sapphire.
Omega Speedmaster, Hesalite vs. sapphire. The Speedmaster is notable for offering customers the option for a Hesalite or sapphire crystal, uncommon nowadays, but a tribute to the original Moonwatch.
Life In Plastic
Plexiglass, or polymethyl methacrylate (PMMA), also called acrylic or one of its trade names including Hesalite, Plexiglas, or simply “plastic” for shorthand, was developed in 1928 and first commercialized a few years later by chemicals manufacturer Rohm and Hass under the Plexiglas trademark. It was discovered and/or commercialized around the same time by different companies and scientists in Germany, the U.K., and the U.S. Hesalite, most known for its use by Omega in the Moonwatch, is simply a form of plastic that has properties better optimized for its use as a watch crystal.
Vintage Rolex Submariner and Day-Date with acrylic crystals.
The new compound had properties, especially compared to normal glass, that made it immediately useful: flexibility, lightweight, high clarity, and shatter resistance, along with its relative ease of manufacturing (read: cheaper to make).
Early ad for an “unbreakable crystal.” Image: courtesy of Vintage Watch Straps
During World War II, acrylic glass found all kinds of uses in submarines, aircraft, weapons, and – most importantly, of course – watches. Long gone were the days of trench watches with glass crystals that might shatter if not for those cages or covers to protect the fragile crystal. These glass crystals had already been increasingly replaced when “unbreakable crystals” were first introduced in 1915. These unbreakable crystals were made from celluloid, an early, transparent plastic that had the well-known benefits of plastic – moldable, flexible, cheap. Unfortunately for celluloid, it’s also highly flammable and shrinks at cold temperatures. When acrylic came along, it was a clear improvement.
After World War II, acrylic crystals became the standard for the watch industry. Vintage Rolex, Patek, Seiko, and pretty much everything in between used plexiglass. Nowadays, this is even seen as part of the charm of these old watches.
A look at the acrylic crystal on a vintage Submariner.
“Acrylic crystals are such an integral part of the design for vintage watches,” says Greg Petronzi, a watchmaker specializing in vintage Rolex. “Personally, there’s a charm in acrylic that you can’t get out of sapphire.” To Petronzi, a vintage Submariner dial can’t truly be appreciated unless it’s under the right crystal, one that captures the original intent of those who designed the watch.
He’s so passionate about acrylic that he founded TrueDome, which has painstakingly reverse-engineered vintage Rolex “Superdome” crystals for popular vintage sports models. For enthusiasts, the domed, plastic crystal of these vintage tool watches helps set them apart from the flat sapphire crystals more common in modern watches.
Vintage 5513 with a TrueDome plastic crystal, designed to replicate the feeling of an original Rolex crystal. Image: courtesy of TrueDome
Some of this charm also comes from acrylic’s perceived shortcomings as the “old” crystal, but these vintage watches were still robust and purpose-driven.
“Vintage watches became icons because they were quite literally instrumental – unlike their modern, more ornamental counterparts,” says Lorenzo Ortega, co-founder of Lorier. As something of an ode to this heritage, Lorier has always opted to use Hesalite crystals in its vintage-inspired watches.
“Hesalite has an inimitable warmth and clarity. It’s functionally, rather than superficially, robust. Compared to sapphire, it’s also much less likely to shatter,” Ortega added. Compared to modern sapphire crystals, it’s a softer, more gentle look.
Plastic crystals have some drawbacks. On the Mohs hardness scale, they’re about a 3. If you’ve ever tapped one with your finger, you know it doesn’t feel much stronger than your fingernail (2.5 on the hardness scale). Acrylic also scratches easily. But, it’s flexible and tends to bend but not break; those surface-level scratches can also be buffed out with a dab of Polywatch or toothpaste. While acrylic crystals have largely been swapped for sapphire by modern manufacturers, acrylic apologists like Ortega, Petronzi, and – I’ll admit – me, insist that plastic crystals can still stand up to modern wear and tear.
“People don’t realize that a good acrylic can be really robust,” Petronzi said. “For example, a vintage Sea-Dweller or Submariner crystal is a lot thicker than a 1016 [Explorer] crystal.”
How To Make A Plastic Crystal
Astronaut Gary Cooper’s Speedmaster with its scratched-up acrylic crystal. Image: courtesy of The Smithsonian
Acrylic’s flexibility makes it easy and inexpensive for manufacturers to work with. You start with a big sheet of plastic that can then be punched out, formed, polished, and shaped – think of the way you might work with a big sheet of cookie dough during the holidays. Still, serious manufacturers will experiment with different polymers to see how they affect different characteristics.
“With TrueDome, I tested five or six polymers,” Petronzi said of his experimentation to replicate the feel of those vintage Rolex crystals. “Each variation yields differences in optical clarity, modus of elasticity, shock resistance.” While still cheaper than working with sapphire – which we’ll get to – Petronzi says acrylic is “easy to make cheap, but expensive to make it right.” It can easily cost five figures and a year of R&D to make one new crystal reference (TrueDome now has six references for various different vintage Rolex models).
Modern Hesalite Speedmaster.
“We’d take measurements of original Rolex crystals and find there were variations in sidewall thickness, depth of arch, inner and outer diameter,” Petronzi said. To complicate matters even more, old acrylic shrinks a bit, so you can only get a rough measurement and then go through trial and error – often at the level of hundredths of a millimeter – until the look and fit is right.
Because of these properties, watchmakers today sometimes aren’t used to acrylic crystal. Unlike sapphire, acrylic is typically either press-fit into a case or uses a tension ring. With sapphire, the crystal is cut and that’s it; because of acrylic’s properties, there can be more variation in tolerances.
Mineral Rights
Seiko 5 Sports with the brand’s Hardlex mineral crystal.
By the mid-20th century, more brands started using mineral crystals on their watches. Made from silica, they have more optical clarity and scratch resistance than acrylic. While they might be more prone to shattering upon hard impact than acrylic, mineral crystals are generally more durable in everyday use.
Even today, mineral crystals are something of an in-between, often used on mid-range watches. The Seiko 5 Sports uses the brand’s proprietary Hardlex mineral crystal; many watches from Casio, Timex, and Citizen follow suit.
Eyes Like Sapphires
Sapphire crystals on the most recent Rolex Submariner references.
In the 1970s, as it became easier and cheaper to make sapphire synetically, watch manufacturers began adopting sapphire crystals as a replacement for acrylic. Rolex started using sapphire in the late ’70s, beginning with the Oysterquartz and slowly rolling it out to other model lines through the ’80s.
While sapphire wasn’t broadly adopted by the watch industry until the late 20th century, Jaeger-LeCoultre began using it in some watches as early as 1929, like in this Duoplan.
Sapphire has many functional benefits over acrylic crystal: it’s harder, scratch-resistant, and durable. On the Mohs hardness scale, it’s a 9, just below diamond (10). Sapphire has some drawbacks. It’s more reflective and doesn’t have the clarity of acrylic; its hardness also makes it more difficult to work with. If it does get a scratch, it can’t be easily polished out as with acrylic. But most companies and consumers today seem to think its benefits outweigh any shortcomings.
In fact, the material costs of acrylic and sapphire aren’t that different. Instead, the higher cost of sapphire comes down to the tooling and process of producing sapphire crystals. It’s because sapphire is so hard (only diamond is harder), that it’s more expensive to work with.
“Sapphire has to be diamond cut, and then these tools have to be changed after cutting 20 to 25 crystals,” says Wesley Kwok, founder of Nodus Watches. Compare this to acrylic crystals, which can be quickly cut out of a plastic sheet like a Christmas cookie.
In the middle, the Milgauss’ famous glace verte, or green crystal.
Sapphire for watch crystals is synthetically grown into a fist-sized boule that looks like frosted glass before it’s cut and polished into the desired size and shape. We as customers only see the final shape: flat, box, double-domed, and other variations. But for manufacturers, there are two basic ways to cut a sapphire crystal.
“There are two fundamental shapes: flat and boxed,” Kwok said. A box crystal shaves more out of the inside of the crystal. Kwok provided diagrams to illustrate the difference:
Image: courtesy of Nodus Watches
The main benefit of a box crystal is that it allows the hands to sit further up into the crystal cavity; it’ll often look like the hands are floating in the middle of the sapphire. The Black Bay 58’s domed sapphire crystal is just one example.
“This also becomes increasingly important with movements that are ‘top heavy,’ where the bulk of the thickness on the movement is on the top half, such as on the Miyota 9075,” Kwok said.
But, these boxed crystals are more costly. First, they use more sapphire: a standard flat crystal might use two or three millimeters of sapphire; a boxed crystal uses about twice as much. A CNC machine is also required to shave and shape the inner crystal, adding to the manufacturing cost.
The domed crystal of the Tudor Black Bay 58 allows the hands to sit further into the crystal cavity.
The primary functional drawback of sapphire compared to acrylic crystal is that it doesn’t have the same clarity. This is why layers of anti-reflective (AR) coating are usually added to the inside of the crystal, often in multiple layers. AR coating is designed to “cancel out” the light waves reflecting off the crystal to minimize reflection and glare.
These AR coatings are often optimized to minimize yellow and red reflection. Since blue sits on the other side of the visible light spectrum, AR coatings typically have a blue tint (or green, as in the case of the Rolex Milgauss 116400GV). Blue is also less distracting to the human eye – some people even like the tint.
The AR coating on the Nodus Trailtrekker gives it a slight blue tint.
Another common argument against sapphire is that, while it’s hard and won’t scratch or break, if you smack it hard enough, it can shatter.
“If you smack a high-quality acrylic crystal it’ll bend and absorb that energy, whereas a sapphire crystal will either shatter or not,” Petronzi of TrueDome said. Others feel the concern of shattering a modern sapphire crystal is overblown.
“We’ve had exactly one come back with a broken crystal, and it was dropped on a hard tile,” Kwok said. “So to me, that’s not a great argument.”
Acrylic Vs. Sapphire: Clearly A Matter Of Taste
Most will concede that modern production methods make both sapphire and acrylic crystals more than serviceable for the modern wear of banging away at a keyboard or running an annual Turkey Trot. Some acrylic apologists are even skeptical that sapphire is that much better than plastic.
“To be honest, I think it’s marketing more than reality nowadays,” Petronzi said. “I’m not sure [sapphire] is actually that much better.” While the upfront tooling and costs of working with sapphire are expensive, it can pay off on the back end. Once you cut sapphire, you’re done – it isn’t subject to the shrinking and flexing of plastic that can make it trickier to work with or find the proper fit. This also means the reject rate during quality control also tends to be lower. Petronzi said he’s talked to old Rolex watchmakers who said they had three different versions of the same crystal for the same reference, and they never quite knew which would fit the watch on their bench. In a world of cost-cutting, efficiency, and quarterly earnings, that might not work today.
Still, the modern benefits of sapphire are real and don’t require stocking up on Polywatch. Much like the watch itself, crystal preference comes down to the individual.
“Personally, I prefer sapphire because I’m rough on my watches, but it’s a matter of taste,” Kwok of Nodus said. With taste in watches feeling as broad as ever, there’s plenty of room for both acrylic and sapphire crystals, as companies like TrueDome, Nodus, and Lorier have proven.
A Clear Future
Nodus Contrail GMT, featuring a bezel with a sapphire insert.
Most large watch brands continue to use sapphire crystals, with the Omega Speedmaster Moonwatch and its Hesalite crystal perhaps the most notable exception. (Of course, there’s also the sapphire Moonwatch option.)
But, crystals are an exciting area of development for small brands and companies experimenting with both acrylic and sapphire. For example, Petronzi pointed to Swiss brand Laventure, as well as more affordable options from Baltic and Lorier, as exciting options for enthusiasts who prefer the warmth of vintage plastic crystals. That said, this is mostly limited to smaller and independent brands.
“I’d love to see a bit more experimentation from the bigger brands,” Petronzi added.
TrueDome’s D19 crystal, designed for vintage Submariner references. Image: courtesy of TrueDome
For Petronzi, meanwhile, TrueDome was born of a different need, but one that’s equally important for vintage collectors: a diminishing supply of original plastic crystals. Large brands like Rolex have become increasingly strict on access to service parts, making it difficult for collectors who need a replacement crystal for their vintage Rolex. He acknowledges that it’s a niche concern, but for the hardcore collector community, it’s important.
“So many enthusiasts understand that having an original crystal really affects the design of their watch,” he said. “I’m going to make a crystal for them.”
Meanwhile, Kwok said he and Nodus are hard at work figuring out how to engrave sapphire bezel inserts so they can be filled with luminous material to provide a stronger lume signature than current methods. It’s these types of ideas, applied to old or well-known materials, that continue to make watches exciting.
“Aside from the visual quirks, the distinction between the two is largely philosophical,” editor Cole Pennington once wrote when comparing the sapphire and Hesalite Speedies. “Sometimes the philosophical waffling between two ideologies is far more stressful than just sitting down and choosing one.”
Whatever you decide is a matter of personal taste, and you can’t go wrong either way.
Hodinkee
