Sapphire or mineral crystal: the difference that really matters
Sapphire crystal is roughly ten times more scratch-resistant than mineral glass: it is the part that decides whether your watch still looks good in five years. It is also the detail manufacturers cut first when they want to lower the sticker price.
The three types of glass found on a watch
Acrylic (plexiglass)
The cheapest. It scratches so easily that it marks in a pocket, but it has one advantage: scratches polish out with a paste. It is found on vintage watches and at the very bottom of the range.
Mineral glass
Tempered glass, around 5 on the Mohs hardness scale. It is the standard on cheap watches. It takes small knocks, but it scratches on contact with sand, a key or a door frame, and the scratches are permanent.
Synthetic sapphire
Corundum crystallised in a laboratory, hardness 9 out of 10, just behind diamond. In everyday life, almost nothing scratches it: not keys, not concrete, not a car door. Its only weakness is that it is more brittle than mineral glass in a violent frontal impact. It is the glass used on every Modora watch.
Why it is the number one criterion on an affordable watch
A movement can be repaired, a bracelet replaced, a case repolished. The glass, however, is what you look at fifty times a day. A watch with a scratched glass looks worn out even if it runs perfectly. On a tight budget, a watch with a decent movement and sapphire crystal will age better than one with a more prestigious movement but mineral glass.
How to know which glass a watch has
- Read the spec sheet. A manufacturer using sapphire always says so. When the mention is missing or vague ('resistant glass', 'high-hardness glass'), it is mineral.
- The water drop test. A drop placed on sapphire keeps a domed, compact shape; on mineral it spreads out more. It is not foolproof, but it is a good indication.
- The sound. Tapped lightly with a fingernail, sapphire sounds clearer and higher than mineral, which gives a dull sound.
- The price. Below 100 francs, sapphire is very rare. Between 200 and 400 francs it should be systematic: if it is not, the money went somewhere else.
Anti-reflective coating changes the reading
Bare sapphire reflects light slightly more than mineral glass. An anti-reflective coating, applied on the inner face or on both faces, removes most of those reflections and gives the impression that the dial floats with no glass above it. That is what separates a watch that is pleasant to read in full sun from one where you have to turn your wrist to see the time.
Caring for a sapphire crystal
It needs nothing special. A microfibre cloth, slightly damp if needed, is enough. Avoid abrasive products and ammonia-based glass cleaners, which do not attack sapphire but can reach the seals and the anti-reflective coating. If your sapphire does end up scratched, it met something very hard, like sandpaper or a sharpening stone: a watchmaker will replace it, and the operation stays inexpensive.
Frequently asked questions
Can a sapphire crystal break?
Yes, in a violent direct impact on the glass, for example falling onto a concrete edge. It is highly scratch-resistant but more brittle than mineral glass, which absorbs impacts better.
Synthetic or natural sapphire, is there a quality difference?
No, chemically it is the same material. Synthetic sapphire is even purer and perfectly transparent, which the natural stone is not.
Do all Modora watches have a sapphire crystal?
Yes, across the whole catalogue, including the models under 300 francs. It was a decision from the start: we would rather save on advertising than on the glass.
To understand the other key part of an affordable watch, read our guide on the Seiko NH35 movement.
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