Omega Speedmaster - Hesalite vs Sapphire
If you're reading this article, chances are you've followed the same journey trodden by many before and found yourself considering an Omega Speedmaster Moonwatch. And so you should; it's a brilliant watch that offers unrivalled heritage and surprisingly good value.
But there's a sticking point, and it's one that, if you buy a Moonwatch, will stare at you every single day you wear it. And it's this: should you get one with a Hesalite crystal—or a sapphire one?
A watch needs a crystal for a singular reason: to protect the delicate innards from harm. Different watches are manufactured to withstand different kinds of damage, from the dust and moisture of day-to-day wear all the way through to impact and water pressure from specialist use, and the design of crystal must reflect that. But what sets the crystal apart from the rest of the watch's case is the dial that lies beneath it, dictating a specific requirement that makes it unlike any other component in watchmaking: transparency.
For the very first watches, pocket watches, the obvious material of choice was glass. Used in windows from as early as Roman times, and in jewellery for nearly four millennia, it does everything that could possibly be required of it: it can be formed into precise shapes, it's hard, it's easy to produce—and of course, it's transparent. But it has one major flaw: fragility.
Two Omega Speedmasters, but which one is better?
This is fine for a pocket watch. It sits safely in a pocket, often behind a protective metal cover. But for a wristwatch, sat proud and exposed on the wrist, being swung and knocked and scuffed? Glass wouldn't make it to the end of the week. With tough tempered glass not yet available, wristwatch makers had to look elsewhere, to polymethyl methacrylate—also known as acrylic.
Acrylic, the versatile plastic behind everything from hard contact lenses to paints, offers a considerable benefit over glass when used as a watch crystal: toughness. While not as hard as glass and therefore less scratch resistant, acrylic offers twice the resistance to fracturing—and any light marks can simply be polished out. It's propensity to crack instead of shatter on failure is also beneficial, maintaining its structure and therefore reducing the overall damage to the watch. It's why it's used in submarine windows. Acrylic comes under a multitude of a trademarks, including Plexiglass, Perspex and, of course, Hesalite.
This toughness was the motivation behind NASA's decision to retain acrylic for the Speedmaster's crystal when it was selected as the official watch of the Apollo program. Omega did offer to swap the Hesalite out for a newer, harder material—sapphire crystal—however NASA determined that that the failure characteristics of acrylic were favourable to the propensity of sapphire to shatter in the same way as glass. The consequence of a thousand tiny shards of very sharp sapphire floating in zero gravity drove the decision to stick with the tried and tested Hesalite.
Acrylic—or Hesalite—on the left, sapphire on the right
But we get ahead of ourselves. What is a sapphire crystal?
Well, it's exactly what it says it is—sapphire, a variant of the crystalline form of aluminium oxide known collectively as corundum. Corundum grows naturally in a variety of colours; when it's red, it's known as ruby. Any other colour is sapphire. But the corundum used to make watch crystals isn't natural—it's manufactured in a lab. This synthetic sapphire—which is also used to make the rubies found in most watch movements—has a hardness beaten only by diamond, and a toughness some ten times greater than glass and four times greater than acrylic.
We tested the difference between acrylic and sapphire with two test crystals: when we scratched the acrylic with a hard, sharp object, it scratched easily—that's evidence of the poor hardness. We did the same to the sapphire and—nothing. It was absolutely fine. The tight, crystalline latticework holds fast.
The Hesalite is smooth while the sapphire has a step
That solves it then. You should get your Moonwatch with the sapphire crystal. Right? Well, perhaps not, because sapphire does have a few flaws. Take a look at the two Speedmasters side by side; see how the acrylic is gently domed, whereas the sapphire has a more rounded, artificial-looking step?
This is down to the manufacturing process. Where acrylic can be formed into complex shapes with ease, synthetic sapphire must be machined and then polished from a solid block. Being right up there near diamond on the hardness scale makes that a very tricky and lengthy process. This is why most modern watches, like Rolexes, have flat sapphire crystals.
The issue of machining the sapphire goes further still: the acrylic crystal is very thin around the edge, while the sapphire is a lot thicker. As this is where the crystal is seated in the case, the same torsional strength needed for the face of the crystal is not required, a thin edge providing enough strength in compression to maintain structural integrity.
Which crystal do you prefer?
Unfortunately, the machining process of the sapphire crystal is too likely to cause a fracture if milled that thin, so the shape maintains the same thickness throughout. With the crystals face on, we can see that this thick edge produces a milky band around the edge of the sapphire that isn't present on the acrylic. The Hesalite crystal is clear edge to edge, while the sapphire crystal has a milky band around the circumference.
So, to the question of Hesalite versus sapphire: it all comes down to what's most important to you. The authentic vintage feel of the acrylic mirrors what the crew of Apollo 11 wore on that historic journey to the moon, but it will pick up marks, whereas the sapphire offers increased peace-of-mind against knocks and scratches, but its shape isn't as organic and has that milky ring. There's a reason why Omega, unlike almost every other brand, still offers both variants of the same watch, and that's because the decision hangs in the balance; only you can decide.
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