Sapphire, mineral and acrylic crystals

There are three crystal materials in common use, and choosing between them is not a matter of one being better. It is a three-way trade: sapphire resists scratches and shatters, acrylic shatters last and scratches first, and mineral glass sits awkwardly in the middle while being the cheapest to replace.

If you want the practical answer before the detail: sapphire is what you want on a watch you wear daily and want to look unmarked in ten years, acrylic is what you want on a vintage piece or anything you will genuinely abuse, and mineral glass is what you get on watches where the crystal was not the place the budget went.

The three materials at a glance

Hardness below is on the Mohs scale, the mineralogical scratch ranking. Impact behavior is what happens when the watch meets a doorframe corner rather than a grain of sand.

Synthetic sapphire

Synthetic sapphire is single-crystal aluminum oxide, chemically the same material as the gemstone, grown from molten alumina by processes such as flame fusion or crystal pulling and then sliced and polished. It has been produced industrially since the early 20th century, which is also why the jewels inside a movement are cheap.

Its hardness is 9 on the Mohs scale, and only a handful of materials sit above it: diamond, silicon carbide and a few engineered abrasives. The point that actually matters for daily wear is that ordinary environmental grit is largely quartz sand at Mohs 7. Quartz will happily scratch mineral glass and acrylic and can do nothing at all to sapphire. That is the whole case for the material, and it is a strong one.

The weakness is that hardness and toughness are different properties. Sapphire is brittle, and a hard, concentrated impact on a corner or edge can chip a crystal or crack it outright, where acrylic would have taken a dent and carried on. Sapphire also has a stiff, glassy surface that will not absorb a blow, so a fall onto tile that leaves the case unmarked can still leave a crystal in pieces. This is the same hardness-versus-toughness trade covered in the ceramic section of our guide to case materials, and for the same physical reason.

Sapphire is also the most expensive of the three to replace, especially in a shaped, domed or coated form.

Mineral glass

Mineral glass is a hardened silicate glass, which is to say ordinary glass chemistry that has been strengthened, usually by thermal tempering, by chemical ion exchange, or both. Its Mohs hardness is generally quoted somewhere around 5 to 6.

That number puts it below quartz sand, which means it does scratch in ordinary life: beach grit, a set of keys in a pocket, a steel worktop edge. The scratches are usually fine and shallow rather than the deep swirls acrylic collects, but they cannot really be removed. Polishing glass is possible in principle and almost never worth doing on a watch, because you distort the optics before you remove the mark.

Hardened variants are worth knowing by name where a maker uses them, Seiko's Hardlex being the most familiar. Makers rarely publish comparable hardness figures for them, and they are usually marketed for impact resistance rather than for scratch resistance, which is the more defensible claim: chemically strengthened glass has a compressed surface layer that resists crack initiation well.

The genuine argument for mineral glass is economic. It is cheap to make, cheap to stock and cheap to fit, so on an inexpensive watch a broken crystal is a minor annoyance rather than a decision about whether the watch is worth repairing.

Acrylic, plexiglass and hesalite

Acrylic is polymethyl methacrylate, the same transparent plastic sold as Plexiglas or Perspex, and in watchmaking often called hesalite after the trade name Omega uses. Its Mohs hardness is around 3, which means essentially everything scratches it, including a shirt cuff over enough time.

The reason people still choose it, and the reason it commands real affection, is that its scratches are reversible. A tube of plastic polishing compound and two minutes of thumb work will take a hazy acrylic crystal back to clear. No other crystal material offers that, and it changes the ownership relationship completely: marks are a maintenance task rather than damage.

Acrylic also fails gracefully. It is tough rather than hard, so an impact that would shatter sapphire will typically leave a dent, a bruise or a network of fine cracks called crazing. It very rarely produces the sharp fragments glass and sapphire do. That is precisely why Omega has kept a hesalite crystal on the standard Speedmaster Professional throughout its association with crewed spaceflight, the stated logic being that a crystal which crazes rather than shattering does not fill a cabin with fragments.

Optically it is the warmest of the three. Acrylic has a lower refractive index than sapphire, which means less internal reflection and a softer, less glassy look, and a domed acrylic crystal over a vintage dial produces a distortion at the edges that many collectors specifically want. Its drawbacks beyond scratching are that very old acrylic can yellow slightly, and that it is attacked by some solvents, so keep alcohol-based cleaners and perfumes off it.

Why the Mohs scale misleads

The Mohs scale is ordinal, not linear. It was built in 1812 by ranking minerals by which one scratches which, so it tells you the order and nothing about the size of the gaps.

The gap between the numbers is wildly uneven. Measured on an absolute indentation scale, the step from corundum at Mohs 9 to diamond at Mohs 10 is enormous, far larger than the whole distance from 1 to 9. So "sapphire is 9 and mineral glass is 6" does not mean sapphire is half again as hard. It is several times harder in absolute terms.

Anti-reflective coating

Bare sapphire reflects a noticeable amount of light, which is why an uncoated sapphire crystal can look like a mirror outdoors. Anti-reflective coating is a stack of very thin transparent layers, typically metal oxides or magnesium fluoride, applied by vacuum deposition and engineered so reflections from the layer boundaries cancel each other out. Each layer is a fraction of a micrometer thick.

The decision that affects you is which side it goes on.

• Inside only. The coating sits on the underside of the crystal, protected from the world. It kills a good share of reflections and lasts the life of the crystal. This is the conservative, sensible choice, and it is what most manufacturers do.
• Both sides. Legibility becomes remarkable: the crystal can genuinely look absent. The cost is that the outer layer is a soft coating on a hard substrate, and it scuffs. Scuffed anti-reflective coating shows as dull patches or a faint purple or blue smear, and it cannot be polished off without stripping the coating entirely.

That is an honest trade rather than a fault. Double-coated crystals are wonderful on a watch that lives a gentle life, and a source of steady annoyance on a watch that gets knocked. The usual sign is a crystal that looks perfect head-on and grubby at an angle.

Domed, flat and box crystals

Shape changes optics, cost and vulnerability.

A flat crystal is the cheapest to make and the least distorting, and it is what most modern sports watches use. A domed crystal curves across its whole surface, throwing off attractive edge distortion and reflections at glancing angles, and sitting proud of the bezel where it is more exposed. A box crystal (sometimes "top hat") has flat sides rising vertically from the bezel with a flat or slightly domed top, which is a vintage look that has come back strongly.

Two practical notes. Distortion at the edge of a domed crystal is a feature to some people and a defect to others, so look at one in person before paying for it. And any crystal standing proud of the bezel is the first thing to hit a wall, which matters more with sapphire than with acrylic.

Identifying, replacing and resealing

Telling the three apart by hand is less reliable than people claim, but a few signals are decent.

• Temperature. Sapphire conducts heat far better than glass or plastic, so it feels distinctly cold against a lip or cheek and stays cold longer. This is the most reliable of the informal tests.
• Sound. Tapping with a fingernail gives a higher, harder click on sapphire, a duller click on glass and a plastic tap on acrylic.
• Water. A drop of water tends to hold a tighter bead on sapphire than on acrylic. It is a real effect and a weak test, because surface films and coatings change it completely.
• Marks. A crystal with swirly, easily visible scratches and no chips is almost certainly acrylic.

The reliable answer, though, is the specification sheet or the reference number. Do not scratch-test a crystal you own with a testing pen, because if you are wrong you have permanently damaged it to learn something the manufacturer would have told you for free.

On cost, expect broad bands rather than figures: an acrylic crystal is one of the cheapest parts in watchmaking and a generic mineral glass one is barely dearer, with the labor costing more than the part. A flat generic sapphire is still modest. A branded, shaped, domed or coated sapphire with a date magnifier fitted to it is a different order of expense and is often only available from the manufacturer, which is worth knowing before you buy a watch you intend to wear hard. Timing that job to coincide with a full service saves a second round of labor and a second pressure test.