Plexiglass/hesalite/acrylic—plastic for watch crystals. Less marketable than sapphire, and on dive watches, more expensive to manufacture. So why even use it?
The short answer: our watches are built to be reliable tools, not precious jewelry. They're made for those who intend to treat them as such, and who aren't afraid of a little wear and tear. And this used to be the norm—watches were actually used as instruments.
So let's start there.
From left to right:
Omega Speedmaster Moonwatch (c.1967-present): As indicated by the name, the first watch on the moon.
Rolex Submariner ref. 6538 (c. 1955-1959): made famous by Sean Connery, as James Bond, in Dr. No, Goldfinger, From Russia with Love, and Thunderball.
Tudor Submariner ref. 9041/0 (c. 1975-1983): used by French Navy ("Marine Nationale") divers/combat swimmers.
These icons used plastic crystals (in fact, the Speedmaster still has the option to this day); they looked great and could take a beating. But since their time, priorities have shifted, with watches viewed more as status symbols. So it makes more sense for the luxury-oriented industry to shift towards hard-to-scratch sapphire from hard-to-break plexi (without mentioning the marketability of the word "sapphire" itself).
But the difference in form and function remains. Plexi is the more transparent material. It has less glare, because plexi tends to refract more than it reflects (compared to sapphire or mineral glass). It will sparkle rather than mirror, which is why the term "warmth" is often used by vintage watch aficionados to describe plexi's optical qualities, as opposed to the cold reflections of sapphire or glass. It's also harder to smudge.
"Given the choice between acrylic or sapphire, I’ll always choose acrylic for its warmth..."
Structurally, plexi is more flexible, which makes it more resistant to impact. And if it does break, it's much more likely to crack rather than shatter, which better protects the inner components of a watch. It's true that this flexibility makes plexi easier to scratch (sapphire is the opposite; it's much harder to scratch but more prone to shatter). But the good news is that marks can easily be buffed out with virtually any polishing compound.
It's that easy: scratches before and after a quick application of Polywatch.
Since we're romantic nostalgics, the choice to go with plexi was obvious, but it actually wasn't easy. Mounting a plexi crystal on a dive watch is quite difficult and has become somewhat of a lost art. Most case suppliers simply couldn't do it, having been accustomed to sapphire and mineral glass for so long (they're much easier to work with, because they only require a plastic I-ring on the outer diameter of the crystal).
It took some trial and error, but we were able to successfully adopt a mechanism known as the "armored glass" method, used by Omega in the 1950s and 60s, that deploys a steel tension ring on the crystal's inner diameter. It's more expensive to make it this way, but to us, the cost was more than worth it.
Armored glass: using a steel tension ring (encircled) for water resistance
We love plexi because it allows us to time travel a little bit. But we use it not only for the sake of nostalgia; it's not just to call back to an era of watches as instruments. On its own, plexi still looks fantastic and is still extremely practical. It's just as good now as it was back then.