Sapphire crystal

A sapphire crystal is a watch glass cut and polished from synthetic corundum, the same grown material used for jewel bearings. It rates 9 on the Mohs hardness scale, above almost everything a watch meets in daily life, including steel, keys and sand, which is mostly quartz at Mohs 7. That is why a sapphire crystal can spend years on a wrist and still look new.

How it is made

The material is aluminum oxide grown as a single crystal, chemically the same as the gemstone but colorless, since the blue of a natural sapphire comes from trace iron and titanium that a watch crystal does not contain. It melts at around 2,050 degrees Celsius. Boules are grown from the melt by flame fusion, the process Auguste Verneuil published at the start of the twentieth century, or by pulling, then sliced and polished with diamond because little else will cut them. Most of what a sapphire crystal costs is machining rather than material.

The numbers that matter

• Hardness. Mohs 9, around 2,000 on the Vickers scale, against roughly 700 HV for hardened steel and about 1,100 HV for quartz sand. Everyday grit cannot mark it.
• The scale misleads. Mohs is ordinal: it ranks minerals by which scratches which and says nothing about the gaps. The step from 9 to 10 is larger than the whole distance from 1 to 9.
• Refractive index about 1.77. Roughly 8 percent of light reflects at each surface against about 4 percent for glass, which is why an untreated crystal looks like a mirror outdoors.
• Density and heat. About 4 grams per cubic centimeter against 2.5 for glass, and a far better conductor of heat, which is why it feels cold against a lip and stays cold.

Trade-offs against other crystals

Hard is not the same as tough. Sapphire is brittle: a sharp knock on the edge chips it, and a heavy impact cracks it where a softer material would give.

Acrylic, the plastic used on most watches until the 1970s and still on some reissues, scratches if you look at it wrongly, but it takes a knock without shattering and light scratches polish out at home. Mineral glass sits in between, harder than acrylic and cheaper than sapphire, but it neither resists scratches nor polishes out. Our three-way comparison sets out which failure you would rather live with, and is sapphire worth it answers the buying question.

How to read the spec

An anti-reflective coating fixes most of the glare. Applied to the underside only, it is protected from wear and lasts the life of the crystal. Applied to both faces, it gives the striking clarity of some dive and pilot watches, but the outer layer scratches and can eventually look patchy.

Watch the wording. "Sapphire coated", "hardened mineral" and names such as K1 describe mineral glass with a surface treatment, and are not sapphire. Nor is sapphire unscratchable: corundum grit in sandpaper and some soils, silicon carbide, and a diamond on another finger will all mark it, so a scratch is unusual rather than proof of a fake.

Thickness, shape and the cyclops

Thickness is rarely published and worth inferring. A dress watch carries roughly a millimeter to a millimeter and a half of sapphire; a 300 meter diver carries two to three, because the crystal is the largest unsupported area in the case; watches rated for extreme depths pass 5 mm, where the crystal becomes a structural part. ISO 6425 specifies no material, only tests the finished watch passes: see dive watches and ISO 6425.

Shape changes optics and exposure. Flat is cheapest, least distorting and easiest to coat internally. Domed curves across the whole surface, throwing distortion at the edges and standing proud of the bezel where it meets the door frame first. A date magnifier, the cyclops on a Submariner being the familiar case, is a separate lens cemented to the outside and magnifying about two and a half times: it scratches on its own account and is not part of the crystal.

In practice

If a sapphire crystal is damaged, the crystal itself is usually a modest part of the repair bill. Fitting it correctly, replacing the gasket and pressure testing the case afterwards is where the work lies, and a crystal changed without a pressure test is a water resistance problem waiting to happen. A chip at the edge is not cosmetic either, since the crystal seals against a gasket at its rim.

Polishing is not an option, since it needs something harder than the workpiece and only diamond qualifies, so a scratched sapphire crystal is replaced rather than repaired. Indicative US figures for replacement run from about $150 to $450 before tax, depending on whether the part is generic or a shaped and coated piece from the manufacturer. Timing that work to coincide with a service saves a second round of labor and a second pressure test.