Sapphire vs Mineral Watch Crystals: Which Should You Choose?

The crystal is the part of a watch you look at most and think about least. It is also the component most likely to spoil the watch's appearance, because a scratched crystal is visible from every angle and cannot be polished out on most modern watches.

There are three materials in common use, and the trade-off between them is not a simple hierarchy. Sapphire is the hardest. Mineral glass is more shatter-resistant. Acrylic is the softest but the only one you can repair yourself. Which is right depends on what you are likely to do to the watch.

Hardness and toughness are different things

This is the distinction that makes the whole subject make sense.

Hardness is resistance to scratching — to something being dragged across the surface. It is measured on the Mohs scale, where diamond is 10.

Toughness is resistance to fracturing — to a sharp impact. A very hard material is often brittle.

Sapphire is exceptionally hard and comparatively brittle. Mineral glass is less hard and less brittle. Acrylic is soft and extremely difficult to break. So the question is not "which is best" but "am I more likely to scratch this watch or knock it against a wall?"

Sapphire

Synthetic sapphire is crystallised aluminium oxide, grown in a furnace and cut with diamond tooling. On the Mohs scale it sits at 9, just below diamond. Almost nothing you encounter in daily life will scratch it — steel is around 4.5, most stone and concrete around 6 to 7, and quartz sand about 7.

What that means in practice: a sapphire watch worn for a decade generally looks like a watch worn for a year, at least across the crystal. That is the entire value proposition, and it is a substantial one.

The drawbacks are real but modest:

• Brittleness. A sharp, concentrated impact on the edge can chip or crack sapphire where mineral glass would survive. This is uncommon rather than theoretical.
• Reflectivity. Sapphire's refractive index is high, so it reflects noticeably more than mineral glass. Good watches address this with an anti-reflective coating. Where the coating is applied to the inner surface only, it cannot wear off — the Seiko Prospex SRPH55 specifies sapphire with anti-reflective coating on the inner surface, which is the better implementation. The Bulova A-15 Pilot uses sapphire with anti-reflective coating too.
• Cost. It is harder to make and harder to machine.

That last point has changed considerably. Sapphire used to mark the boundary of a serious watch; it now appears well under €200. The Casio Edifice EFB-710D, a 41mm chronograph rated to 200 metres, and the Casio Edifice EFR-S108DE at 40mm with 100 metres are both listed with sapphire. At €385, the Tissot Everytime Swissmatic pairs sapphire with a Swiss automatic movement and a 72-hour reserve.

Mineral glass

Mineral glass is hardened silicate glass — ordinary glass, heat- or chemically-treated to improve its surface hardness. It sits around 5 to 6 on the Mohs scale.

It will scratch. Sand, grit, a stone wall, a zip, a set of keys in the same pocket — all harder than mineral glass. Over a few years of regular wear, a mineral crystal accumulates fine surface marks that catch the light.

What it does well is absorb impact without shattering, and it is cheap enough that replacing it is a minor job rather than a decision.

Hardlex, and why Seiko is a special case

Seiko fits its own hardened mineral crystal, Hardlex, across a great deal of its range. It is treated to be harder than standard mineral glass while retaining mineral glass's impact resistance — a deliberate engineering choice for sports and dive watches, where a knock is more likely than a scratch.

Hardlex appears on the Seiko 5 Sports SRPD61K1, the Prospex SRPD21J1 and the Seiko SKA663. It is a reasonable specification for its purpose. It is not sapphire, and on a watch worn daily for years you will see the difference.

Elsewhere, mineral is used where a curved crystal is required, since curving sapphire is expensive. The Bulova Icon 98B316 uses a curved mineral crystal on a 300-metre chronograph, and the Bulova 98B345 a scratch-resistant mineral crystal at 100 metres.

Acrylic

Acrylic — Plexiglas, Hesalite — is a clear plastic, soft at around 3 on the Mohs scale. It scratches if you look at it wrongly.

Its redeeming quality is genuinely useful: you can polish the scratches out yourself, with a plastic polish and a cloth, in a few minutes. Nothing else on this list can be repaired at home. It is also virtually unbreakable and, if it does fail, tends to craze rather than shatter into the movement.

You will find it mostly on vintage watches, on faithful reissues, and on watches where warmth of appearance matters more than sharpness.

Comparison

| | Sapphire | Mineral / Hardlex | Acrylic | |---|---|---|---| | Mohs hardness | ~9 | ~5–6 | ~3 | | Scratch resistance | Excellent | Moderate | Poor | | Impact resistance | Good, can chip | Very good | Excellent | | Reflections | High, needs coating | Low | Low | | Home repair | No | No | Yes | | Typical cost to replace | High | Low | Very low |

How to choose

Buy sapphire if the watch will be worn daily for years. This is the default recommendation. A daily watch meets doorframes, desks and shirt cuffs constantly, and scratch resistance is what preserves its appearance. If two otherwise similar watches differ only in crystal, sapphire is usually worth the difference.

Mineral or Hardlex is a sensible choice for a knockabout sports watch, particularly where impact is more likely than abrasion, and on watches where you would rather the money went into the movement or the water resistance. A Seiko 5 Sports with a 4R36 movement and Hardlex is a better-balanced watch than a hypothetical alternative with sapphire and a lesser movement.

Acrylic is a deliberate, characterful choice for vintage-style watches — and the only crystal you can maintain yourself.

Do not let the crystal override everything else. Water resistance, movement quality and how the watch fits your wrist all affect your experience more than the crystal does. Our guides to water resistance, movements and case sizing cover those.

Looking after any crystal

Take the watch off before rough work. No crystal survives a concrete wall.

Do not stack watches loose in a drawer. Steel cases and bracelets scratch crystals. A pouch or a compartment box costs little.

Clean with a soft, damp microfibre cloth. Dry cloths drag grit across the surface. Avoid solvents on coated sapphire.

A chipped crystal is urgent. A crack breaks the water seal and lets in moisture and dust. Get it replaced rather than living with it.

Sapphire can be replaced. If your favourite watch has a marked mineral crystal, a watchmaker can sometimes fit a sapphire replacement. Ask.

The care and servicing guide covers cleaning, storage and service intervals in more detail. To browse by what you get for the money, start with watches under €500.

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