The Bench · N° 27

Why anti-reflective coatings can scratch before sapphire does

Sapphire is extremely hard, but the optical films applied to it are much thinner and can mark under abrasion. Coating placement determines whether those marks are exposed to daily wear.

Hen & Mills Editorial · 5 min read · 3 September 2026

A sapphire crystal can look scratched even when the sapphire itself is intact. Often the mark is in the anti-reflective coating applied to its surface.

That is not a contradiction. Sapphire and the coating are different materials doing different jobs. The crystal provides the hard transparent barrier. The coating controls reflections.

Where the coating is placed determines how exposed it is to daily wear.

Why sapphire still reflects light

An uncoated sapphire crystal is clear, but each air-to-sapphire boundary reflects some light. On a watch, those reflections can turn the crystal into a mirror and make the dial harder to read.

The effect becomes more noticeable under bright lamps, in direct sun and when the dial is dark. A highly polished dial or a steeply curved crystal can add more visible reflections.

Anti-reflective treatment reduces those reflections so more light reaches the dial and returns to the eye.

How the coating works

Anti-reflective coatings are built from very thin optical layers. Their thickness and refractive properties are chosen so that reflected light waves partly cancel one another.

A simple description is that the coating manages the phase of the reflected light. In practice, manufacturers may use several layers to reduce reflection across more of the visible spectrum.

The result can make the crystal appear to disappear at some angles. It is an optical effect, not a change to the hardness of the sapphire underneath.

The coating is not sapphire

Sapphire rates very highly for scratch resistance. The deposited films above it do not automatically share the same hardness.

They are extremely thin and may be marked by abrasive particles, repeated rubbing or contact with a harder edge. A visible line can therefore sit in the coating without cutting into the crystal.

This is why a supposed sapphire scratch may change or disappear if a damaged external coating is professionally removed. That work should be left to someone who understands the crystal and its treatment.

Inside-only coating

An inside-only coating sits on the surface facing the dial. The sapphire protects it from direct contact with desks, door frames, grit and cleaning cloths.

This arrangement gives a useful reduction in internal reflections while keeping the coating away from most abrasion. The outer sapphire surface will still reflect more light than a coated one, so the crystal may remain visible in difficult lighting.

For many everyday watches, that is a sensible durability-first compromise.

Coating both sides

A coating on the outside surface can reduce the strongest remaining reflection. Combined with an internal coating, it can produce the dramatic clear-crystal appearance seen on some sports and luxury watches.

The trade is exposure. The outer film is now the first surface touched by dirt, sleeves, fingerprints and cleaning materials.

That does not mean it will immediately fail. Modern coating systems can include harder protective top layers. It does mean the visible surface is no longer bare sapphire.

Hard coatings still have limits

Manufacturers sometimes describe an external treatment as hardened or scratch resistant. Those descriptions can be useful, but they do not make every coating equivalent to sapphire.

Durability depends on the complete layer system, its adhesion, the top coat, the test method and the sort of contact it encounters. Fine dust trapped under a cloth can be more abrasive than a clean soft sleeve.

A claim about laboratory hardness does not predict every mark from years of wear. It should be read as improved resistance, not immunity.

Why some coatings look blue or purple

Residual colour comes from the wavelengths the coating does not suppress equally. Depending on the design and viewing angle, the remaining reflection may appear blue, violet, green or nearly neutral.

The colour alone does not tell you whether the coating is good or bad. It reflects the optical design and the compromises made across the visible spectrum.

A strong blue flash can be deliberate. A subtler coating may be chosen when the maker wants the dial colour to remain visually neutral.

Cleaning without creating avoidable marks

First remove loose grit rather than rubbing it across the crystal. If the watch's water resistance is suitable and current, gentle rinsing can help. Otherwise, use a clean blower or a very soft brush with care.

A clean microfibre cloth is safer than a shirt cuff or paper towel. Avoid aggressive polishing compounds and household chemicals unless the manufacturer specifically approves them.

Cleaning cannot prevent every mark, but it can avoid turning a small hard particle into an abrasive tool.

What damage looks like

Coating damage may appear as fine swirls, a coloured line or a patch with different reflectivity. Under magnification, it can look different from a groove cut into the sapphire.

Appearance alone is not always enough for a firm diagnosis. A watchmaker can inspect the surface and advise whether the mark is in the coating, the crystal or material transferred onto it.

Do not start with an abrasive polish. That can make a small defect larger and alter the coating around it.

Service options

Depending on the watch, the practical options may include living with the mark, replacing the crystal or having an external coating removed by a specialist. Recoating is possible in some settings but is not a routine retail service for every model.

Crystal replacement can be costly, particularly when the part has a special shape, gasket arrangement or branding. Ask what is being replaced and whether the new part has the same coating specification.

Choosing the right compromise

For a watch likely to see hard daily use, inside-only coating offers good protection with moderate reflection control. Double-sided treatment gives stronger visual clarity but places an optical film in harm's way.

Neither arrangement is universally superior. The right choice depends on lighting, use and how much visible wear bothers you.

The main point is simple: a marked coating does not prove that sapphire is soft. It shows that the crystal is a system of materials, and its clearest optical surface may also be its most exposed one.