So, you’ve bought a watch with a sapphire crystal. You’ve knocked it accidentally against a wall, a door frame, whatever. And, it’s scratched. But how could this have happened?! It’s sapphire; it’s scratchproof! Why are there scratches on it?
A word or two about sapphire crystal
Watch crystals are made of synthetic sapphire. In terms of the structure, it’s almost identical to natural sapphire. It’s still the same crystalline form of aluminum oxide. Aluminum oxide’s crystalline state, called corundum, exists in various forms. One of these forms is ruby.
The main property of this substance in this state is its unusual hardness. It’s a 9 on the Mohs scale of hardness. For comparison, a diamond is a 10, which is why it’s said that sapphire can only be scratched with a diamond. Rubies are used for bearings in the movement, although sapphires since they also are very resistant to wear and tear, were also used as bearing jewels. If you ever find a blue jewel in any movement, that’s a sapphire.
So, it’s very scratch-resistant. Scratchproof, almost. So why’s that scratch there?
It’s not the sapphire that got scratched
Your watch most likely has a crystal with an anti-reflective coating (AR coating) on the outside or both sides of it. At certain angles, you’ll certainly see the purple-ish or blue tint to the crystal. That’s the AR coating.
A bare sapphire crystal tends to reflect light strongly. In certain lighting conditions and at some angles, this reduces the legibility of the dial.
The problem is that manufacturers often settle for using an AR coating on the outside or both sides, for maximum effect. This improves legibility. However, since an AR coating isn’t tough, let alone scratch-resistant, it’ll develop some wear and tear over time.
So, when you knocked your watch against something, the scratch that appeared isn’t in the sapphire. It’s in the AR coating layer. Sapphire can certainly chip at an impact. It might be hard to scratch, but it’s still brittle.
What to do now?
Well, either you can get that polished, or replace the crystal. The cost of a replacement could be high, depending on the brand. It’s not as simple as replacing a round acrylic or hesalite crystal.
Polishing, on the other hand, will remove the AR coating. This will affect the legibility, but no more ARC means no more scratches.
When buying a watch, carefully look at the technical description. Manufacturers usually provide info about whether the AR coating was used on both sides, or just on the inside. If you feel like you can sacrifice a tiny bit of legibility in favor of resistance to scratches, choose a watch with an AR coating on the inside of the crystal. Otherwise, you’ll need to be careful. Maybe not as cautious as with acrylic or hesalite, but still.
Did you ever have scratches on a sapphire crystal? Let me know in the comments below.