The crystal decides how a watch ages more than any other external part. Three materials cover the market, and the trade-off between them is scratch resistance versus shatter resistance — you get one, not both.
How to tell which crystal you have
Start with the case back: many watches print “SAPPHIRE CRYSTAL”, “MINERAL GLASS” or “HESALITE” there, and a spec sheet or the brand’s product page settles it in seconds. Without a marking, three observations separate them.
- Reflection. Sapphire is usually coated with an anti-reflective layer, so it reflects less and looks almost invisible over a dark dial. Mineral reflects more, and acrylic reflects the most, often with a faint warm tint.
- Dome and edge. Acrylic is normally domed and slightly soft at the edge; sapphire and mineral are flatter with a crisp edge. A strongly domed crystal on a vintage-style watch is almost always acrylic.
- Sound and feel. Tap a fingernail on the crystal: acrylic sounds dull and marks easily with a fingernail; mineral and sapphire sound sharper and do not mark. Do not use a coin or a knife to test — that is how crystals get scratched.
If none of that settles it, a watchmaker can identify the material in a minute, and the answer matters before you try to polish out a scratch.
Sapphire
Synthetic sapphire is the hardest crystal in common use, around 9 on the Mohs scale, and only a diamond or a sapphire-tipped tool will scratch it. It is also the most brittle: a sharp knock on a door frame can chip an edge or crack the glass outright, and replacement is the most expensive of the three. Most sapphire crystals carry an anti-reflective coating, which can itself be scratched even when the sapphire is not.
Mineral
Mineral glass is heat-treated glass, around 5 on the Mohs scale. It resists light scratches for years and then accumulates a haze of fine marks that polish out only partially. It is far less likely to shatter than sapphire and much cheaper to replace, which is why it is standard on mid-range sports watches.
Acrylic
Acrylic (often called Hesalite or Plexiglas) is a plastic. It scratches if you look at it firmly, and it can be polished back with a cloth and a little paste in a minute. It does not shatter into fragments, which is why it was specified for flight and space watches, and why vintage-style pieces still use it.
Comparison
| Crystal | Hardness | Scratches | Shatters | Typical replacement |
|---|---|---|---|---|
| Sapphire | ~9 Mohs | Rarely | Yes, on impact | $60–$150 fitted |
| Mineral | ~5 Mohs | Over time | Rarely | $30–$60 fitted |
| Acrylic | ~3 Mohs | Constantly | No | $20–$40 fitted |
Those ranges are for a standard round crystal fitted by an independent; a shaped or branded crystal costs more, and a watch rated for water needs a pressure test afterwards.
Common mistakes
- Polishing a coated sapphire. The scratch is usually in the anti-reflective layer, and polishing removes the coating in that spot, leaving a visible patch. The safe approach is described in how to polish scratches out of a watch crystal.
- Using toothpaste on any crystal. It is abrasive enough to haze mineral and acrylic and does nothing to sapphire.
- Trusting a listing that says “sapphire” without a marking. Some sellers use “sapphire-coated” for mineral; the case back is the evidence.
- Replacing a crystal without the gasket. The crystal seal is part of the water resistance, and a new crystal with an old gasket leaks — the same logic that applies to any case back that has been opened.
Crystal care that actually helps
Rinse the crystal with water before wiping it: grit on a dry cloth is what grinds a scratch into mineral and acrylic. Use a microfibre cloth, not a sleeve, and never a paper towel. Keep the watch away from keys and coins in a pocket, which is where most scratches happen. On a coated sapphire, clean with a drop of dish soap and warm water rather than an alcohol wipe, because repeated solvent exposure degrades the anti-reflective layer even though the sapphire itself is untouched.
If the watch is rated for swimming, remember that the crystal is part of the seal, not just a window: a chipped edge is a leak path, and it should be checked with the gasket at the same time as any water-resistance service.
Frequently Asked Questions
Can a scratched sapphire crystal be polished?
Usually not. What looks like a scratch is often coating damage, and polishing the coating removes the anti-reflective layer in that spot. Replacement is the normal fix.
Why do expensive watches use acrylic?
For the look and for safety: acrylic does not shatter, and it can be polished by hand. It is a deliberate choice on heritage reissues, not a cost cut.
Does a sapphire crystal make a watch scratch-proof?
No. The crystal resists scratches; the case, bezel and bracelet are usually softer steel and will scratch normally.
Can I replace a crystal myself?
Only with a press and the correct gasket, and only on a snap-fit crystal. On a watch rated for water, a crystal change needs a pressure test afterwards.
The We Know Watches editorial team brings together over 40 years of combined watch collecting, trading, and repair experience. Our editors have owned and handled watches from every major brand — from entry-level Seiko 5s to Rolex, Omega, Patek Philippe, and independent Swiss watchmakers. We’ve bought and sold at auction, worked with authorized dealers, visited manufacturing facilities in Switzerland and Japan, and serviced hundreds of movements ranging from the Seiko 7S26 to the Longines L888. Every guide and review we publish is based on hands-on experience, original research, and consultation with professional watchmakers. We do not accept payment for reviews, and we clearly disclose when we use affiliate links.