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Crystals: Sapphire vs. Acrylic

If you’re reading this, you’re probably aware that most upscale watches feature sapphire crystals. And you probably know that once upon a time, they did not.

It turns out that acrylic crystals predate sapphire by quite a bit.

Sapphire? Acrylic? Let’s explore these two materials a little bit and discover their history, and why one or the other might be better in certain situations.

Let’s begin with sapphire. Now, when you hear the word, do you think of the blue gem? And do you wonder if sapphire as used for watch crystals is related at all? Maybe it’s just a trade name for something else.

Well, it turns out it’s not. It’s the same mineral as the blue gem. The mineral name is corundum – second hardest mineral substance on the planet, next to diamond. Now you know why it’s used for watch crystals. It’s really hard to break. But where the heck do you find all those huge, clear (read, not blue) sapphires from which to make watch crystals!?

Well, you make ‘em. Yes, sapphire watch crystals are synthetic sapphire, grown in a lab for the express purpose of making said watch crystals (this laboratory process first came on the scene in 1902). The resulting boule, as it’s called, is a vulgar (some would say obscene) object about six to eight inches long and maybe two inches in diameter. You get the idea.

From this boule, the crystal makers cut slices and then grind and polish them using advanced lens-making techniques to produce the finished product, ready to install in a watch case.

(Interesting side note – pure corundum would be clear, but impurities make the natural mineral blue – or red, or nearly any other color. That’s why you see pink sapphires, green, yellow, purple, etc. The really red ones are called… rubies. Yes, rubies and sapphires are the same mineral, just with different impurities)

Acrylic, on the other hand, is a sort of plastic first create in 1843. Known to chemist types as polymethyl methacrylate, it’s known by several trade names, Lucite, Acrylite, Plexiglas, Perspex, and Optix among them. The Hesalite crystal used in the Omega Speedmaster Professional moon watch is an advanced form of acrylic.

Being a man-made substance, acrylic can be created in numerous configurations and easily worked into such shapes as are needed.

So why might you want one – either sapphire or acrylic – at the expense of the other?

Well, as I said above, sapphire is one of the hardest minerals known to man. As such, it’s very hard to scratch or break. Plus, it’s structurally stronger than acrylic for a given thickness. That’s why it’s used in dive watches. It can withstand a greater depth, and can take significant abuse in a situation where acrylic would warp and permit flooding of the watch.

Acrylic, on the other hand, is lighter that sapphire, and doesn’t promote glare nearly as much (sapphire is often coated with anti-reflective coating to reduce glare). It can withstand a much heavier blow than sapphire without breaking, although it will scratch. Most scratches, depending on how deep they are, can be polished out with a mild abrasive.

The (NASA qualified for all extra-vehicular activities) Omega Speedmaster Professional has an acrylic crystal for the simple reason that breakage would be intolerable in space. You see, when sapphire does break, it shatters into innumerable tiny needle-like pieces, along with the just-as-sharp bigger chunks. Imagine those floating around in a space vehicle. Nasty…

And the same blow is only going to put a small ding in an acrylic crystal. Much more tolerable in the weightlessness of space.

So when you need tough beyond tough, no breakage under any circumstances, choose acrylic. When you need scratch resistance and structural integrity, choose sapphire.

 

-Ed Estlow

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