Crystal

The crystal is the transparent disc that covers the dial and seals the front of the case. Three materials dominate, and they behave very differently. Synthetic sapphire is single-crystal aluminum oxide and rates 9 on the Mohs hardness scale, second only to diamond among common materials. Mineral glass is hardened silicate glass at roughly 5 to 6. Acrylic, a transparent plastic sold under names such as Hesalite and Plexiglas, sits at around 3.

Sapphire for watches is not mined. It is grown from molten aluminum oxide into a boule, a process descended from the flame fusion method Auguste Verneuil published in 1902, then sawn into discs with diamond tooling and polished. That is why sapphire costs more than glass: the material is cheap and the machining is not.

Whatever it is made of, the crystal is a structural part. It is pressed into the case against a nylon gasket, clamped by the bezel, or held with a tension ring, and it carries the pressure load underwater. Thickness follows the rating: about 1 mm on a thin dress watch, 1.5 to 2 mm on a typical diver, and 3 mm or more, often domed, on watches rated for saturation depths. A dome resists pressure better than a flat disc of the same thickness, which is why deep divers rarely have flat glass.

Hardness is not toughness

Hardness predicts scratching, not survival. Sapphire will shrug off keys, coins and desk edges that would mark mineral glass in a moment, but it is brittle: a sharp impact on the edge can shatter it. Acrylic scratches if you look at it hard, yet it is the toughest of the three in the sense that it deforms and crazes rather than shattering, which is one reason it stayed in use on some professional watches long after sapphire was available. The Omega Speedmaster Professional is still sold with a hesalite crystal for exactly that reason, alongside a sapphire version.

The Mohs scale is ordinal, so the gaps between its numbers are not equal. Measured on the Knoop scale, sapphire comes out around 2,000 and hardened mineral glass around 500 to 600, a difference of roughly four times rather than the two steps Mohs suggests. The practical consequence is a threshold effect: everyday grit and dust is largely quartz, which sits at Mohs 7. That is hard enough to haze mineral glass steadily over a year of wear and not hard enough to touch sapphire. What does mark sapphire is a narrow list, chiefly silicon carbide abrasives, tungsten carbide and diamond.

How to read the spec

Anti-reflective coating matters as much as the material. Sapphire reflects strongly, so most sapphire crystals carry a coating on the underside, on both sides, or not at all. Coating both sides gives the clearest view and the most visible wear, since the exposed outer layer can scuff over years.

Shape affects both price and character. Flat crystals are cheapest, domed crystals distort at an angle and throw the reflections vintage watches are loved for, and box crystals raise a flat top on vertical walls.

Watch the wording, not the noun. "Sapphire coated" or "sapphlex" style names usually describe mineral glass with a thin hard layer on top, which scratches through in time and is not the same thing as a solid sapphire crystal. Seiko's Hardlex is a hardened mineral glass, not sapphire, and the brand says so plainly. The jargon guide collects the rest of these constructions, and watch crystals compared and the three-way comparison go further into which to buy.

In practice

Repairability inverts the hardness order. A scratched acrylic crystal can be polished out at home in minutes with a plastic polish. Mineral glass can sometimes be improved. A scratched sapphire crystal cannot practically be polished and is replaced, which is a routine watchmaker's job but not a free one: budget an indicative $150 to $450 in the US before sales tax, depending on whether the part is generic or a branded item, with more detail in our service cost breakdown. What to do about a marked crystal is set out in scratched crystal.

Fogging under the crystal means moisture has already got in and is condensing on the coldest surface. It is a sealing fault rather than a crystal fault, and it is covered in why there is fog under my watch crystal.

The common confusion

The word does double duty. The crystal you look through has nothing to do with the quartz crystal inside an electronic watch, which is a tiny tuning fork oscillating at 32,768 Hz to regulate the time. A quartz watch with a sapphire crystal contains two entirely unrelated things called crystals, and neither is made of the other's material.

The second confusion is the word scratchproof, which no manufacturer standard supports. Sapphire is scratch resistant against ordinary life and can still be marked by anything harder, which is why sapphire crystals on well-worn watches sometimes carry fine lines. Whether the upgrade is worth paying for is argued through in is sapphire crystal worth it.