The crystal is scratched

There are three crystal materials in ordinary use and they behave nothing like each other. Before you do anything, find out which one you have. The manufacturer's specification page, the owner's manual or the model listing will say. If the watch cost very little and the specification is silent, assume mineral glass. If it is a vintage watch made before roughly the 1980s, assume acrylic.

Check the coating first, because it is usually the coating

This is the single most useful thing on this page. A large share of watches sold with sapphire crystal also carry an anti-reflective coating, and that coating is a stack of vacuum-deposited films measured in fractions of a micron. It is orders of magnitude softer than the sapphire it sits on.

So when an owner says "my sapphire is scratched", what has usually happened is that the coating has been scratched, worn or flaked while the sapphire underneath is exactly as it left the factory. Sapphire is Mohs 9. Almost nothing in daily life is harder. Door frames, keys, desks and car keys are all steel at roughly Mohs 4 to 5. Sand is quartz at about 7. None of those mark sapphire. All of them mark a coating.

How to tell them apart:

• The fingernail test. A real scratch has an edge. Your nail catches in it. Coating damage feels perfectly smooth.
• The angle test. Tilt the watch under a light. Coating damage changes character as you tilt: a colored sheen that disappears in patches, or a hazy area that vanishes head-on. A crystal scratch looks the same from every angle.
• The shape. Coating failure appears as broad cloudy patches, fine crazing, or hairline webs. Crystal scratches are single defined lines.
• The color. Intact anti-reflective coating typically shows a blue, purple or green cast at an angle. Where it has gone, the surface reads as plain clear glass.

Acrylic: polish it yourself

Acrylic, also sold as Hesalite or Plexiglas, is a soft plastic. It scratches if you look at it and it polishes out with almost nothing. That trade is why the Omega Speedmaster Professional is still offered with an acrylic crystal as standard, and why acrylic dominated wristwatches for decades.

The method:

1. Mask the case and bezel with tape if the compound could reach a brushed surface you do not want polished.
2. Put a small amount of a mild plastic polish on a soft cloth. Purpose-made watch crystal polishes exist and cost very little. Metal polishes are too aggressive.
3. Work in small circles with light pressure for two or three minutes at a time. Warmth is normal, heat is not.
4. Wipe clean and check under a light. Repeat if needed.
5. For a deep scratch, very fine wet abrasive paper at 2000 grit or above first, then polish, and accept that you may end up with a very slightly flatter area.

Do not use a powered buffing wheel. It generates enough heat to soften and distort acrylic in seconds, and the distortion cannot be undone.

Mineral glass: usually replaced, sometimes improved

Mineral glass sits around Mohs 5 to 6, with hardened and coated variants higher. It marks readily against sand and against harder steels, and unlike acrylic it does not polish out in any way an owner can do at home.

A watchmaker with lapping equipment can sometimes improve a light scratch on a flat mineral crystal using cerium oxide and a great deal of patience. In practice this is rare, because the labor exceeds the cost of a replacement crystal on almost every watch that uses mineral glass. Expect to be told to replace it, and expect that to be the right advice.

Mineral glass has one behavior worth knowing: it chips and cracks rather than deforming. A hard corner impact that would dent an acrylic crystal can put a crack across a mineral one. A cracked crystal is no longer sealing, so treat it as an urgent water resistance problem, not a cosmetic one. If you already see moisture inside, go straight to our page on fog under the crystal.

Sapphire: replaced, never polished

Synthetic sapphire is aluminum oxide grown as a single crystal, and at Mohs 9 the only common abrasive that cuts it is diamond. Polishing it would require diamond lapping equipment, would take away material unevenly, would destroy any domed profile and any coating, and would still cost more than a new crystal. So it is not done. A scratched sapphire crystal is replaced.

Which is exactly why you should confirm the scratch is really in the sapphire before agreeing to anything. Our comparison of sapphire against mineral and acrylic sets out the full trade, and is sapphire worth it answers the buying question directly.

One caveat that surprises people: hardness is not toughness. Sapphire resists scratching brilliantly and shatters more readily than mineral glass under a sharp impact, because it is brittle. A dropped watch is more likely to lose a sapphire crystal than a mineral one. That is the reverse of what most owners assume.

What it costs

Figures are indicative and move, and a shop outside the US will quote in its own currency with its own tax on top. Independent watchmakers can source generic crystals for many common case shapes, which is where the low end of that range comes from. Brands that restrict parts supply remove that option entirely, and the practical consequence is described in our service cost page and the cost of ownership calculator.

How to avoid the next one

Most crystal scratches happen in three places: doorframes, gym equipment and the inside of a bag where the watch travels loose against keys. Wearing the watch slightly further back on the wrist, as discussed in how a watch should fit, removes a surprising share of the doorframe contacts.

If you are still choosing, decide with the coating question in mind. Sapphire with inner coating only is the durable combination. Sapphire coated on both sides is the legible one, and you should expect to see coating wear within a few years of daily wear. Acrylic is the one you can maintain forever, which makes it a genuinely sensible choice on a watch you plan to keep for decades. The full material comparison is in our guide to watch crystals, and the case-side equivalent in case materials.