FAQ: Why Are There Scratches on My Sapphire Crystal?

 In FAQ

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.

Squale with a sapphire crystal with AR coating
Squale with AR coating on both sides of the crystal.

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.

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Showing 7 comments
  • Avatar
    Ben
    Reply

    I have one sapphire with scratch and I am looking for safe and efficient diamond paste, someone can propose?

  • Avatar
    Amis
    Reply

    Just get it polished, from any certified watch agent, this is if the crystal is not scratched . All Omega now is both side AR coated and rolex is starting the same on some models ONLY.

    • Melvin Hollenberg
      Melvin Hollenberg
      Reply

      Thanks for the insight, Amis. That sounds like a good idea.

    • Avatar
      Carol
      Reply

      If it’s a Rolex watch, how will I know if I can get it polish or must replace the watch screen

      • Melvin Hollenberg
        Melvin Hollenberg
        Reply

        Most Rolex models don’t have AR coating so you can get it polished.

  • Avatar
    Clive
    Reply

    Great article as always. Certainly helped me understand why in certain light/angles one of my watches looks like it’s got misty glass. I suppose having AR re-applied would be restrictive.

    • Melvin Hollenberg
      Melvin Hollenberg
      Reply

      I haven’t heard of re-applying it. The best way would be to have a new crystal fitted with an AR coating.

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