The crystal is the part of a watch you look through every time you check the time, and it is the part most likely to look worn out in three years. It is also the one specification where the expensive answer is unambiguously harder — and still not automatically the right answer.
Sapphire is synthetic corundum, the same crystal as a natural sapphire grown in a furnace rather than the ground. On the Mohs hardness scale it sits at 9, one below diamond and two above quartz — and quartz, at 7, is what sand and most grit is made of. That single relationship is the whole argument: ordinary grit cannot scratch sapphire, and it scratches everything else on this page.
Mineral glass is hardened glass, around 5 to 6 on the same scale, so sand wins. Hardlex is Seiko's own hardened mineral glass, which sits between ordinary mineral and sapphire. Acrylic is plastic, scratches if you look at it, and polishes back out with a cloth. Resin glass is the cheapest of all and turns up on digital Casios.
The short version: sapphire is worth it, and it is no longer expensive — the Timex Expedition North has it at well under half of what a Swiss mechanical costs. But sapphire is brittle where acrylic is nearly unbreakable, and it glares unless it has been anti-reflective coated, which is why some very good watches deliberately do not use it.
The hardness scale, and why 7 is the number that matters
The Mohs scale ranks minerals by which one scratches which: talc is 1, diamond is 10, and each step scratches everything below it. Watch crystals land as follows — sapphire at 9, Seiko's Hardlex between ordinary mineral and sapphire, mineral glass around 5 to 6, and acrylic around 2 to 3.
The useful number is not 9. It is 7, which is where quartz sits — and quartz is the main component of ordinary sand, beach grit and a lot of household dust. Sand is harder than mineral glass and softer than sapphire. That is why a mineral-crystal watch worn at the beach picks up fine scratches and a sapphire one does not, and it is the entire practical difference between the two materials in one sentence.
Sapphire: hard, brittle, and now cheap
Sapphire effectively does not scratch in normal wear. Door frames, desks, car keys, sand — none of it marks the crystal. A sapphire watch looks the same in year ten as it did on day one, which is a bigger deal on a watch you intend to keep than almost any other spec.
What you give up is toughness, which is not the same as hardness. Sapphire is hard but brittle: a sharp impact on the edge of the crystal can chip or shatter it where a plastic crystal would have shrugged and a mineral one might have survived. This is rare in practice, but it is why NASA used acrylic rather than sapphire on the watches it qualified for spaceflight — a shattered crystal inside a spacecraft is a worse problem than a scratched one.
The second cost is glare. Sapphire reflects more light than mineral glass, so an uncoated sapphire crystal can be harder to read in bright sun than a cheaper watch. Manufacturers fix this with anti-reflective coating, usually on the inside face; where the coating is on the outside it works better and wears off.
The news of the last few years is price. Sapphire used to mark a four-figure watch. On this page the Timex Expedition North Field Post Solar carries sapphire with a 100m rating and a screw-down crown, and the Orient Kamasu carries it on a 200m automatic diver. Both sit well under the cost of the cheapest Swiss mechanical, and both are the reason we treat sapphire as a default expectation rather than a luxury on the dive and field roundups.
Hardlex: Seiko's answer, and the reason people complain
Hardlex is Seiko's proprietary hardened mineral glass, used across the Seiko 5 Sports and much of Prospex. It is harder than ordinary mineral glass and softer than sapphire, and it is the single most argued-about specification in affordable watches — because the Seiko 5 Sports SRPD on this page sells at a price where the Orient Kamasu gives you sapphire.
Our honest read: Hardlex is fine, and it is not what we would choose. It resists scratching better than plain mineral, it is cheaper for Seiko to produce and easier to replace, and in daily office wear you may never mark it. But grit still beats it, and when the direct rival at the same money has sapphire, Hardlex is the line on the spec sheet that loses the argument. That comparison is the spine of our Orient pages and of Seiko vs Orient.
Mineral: the sensible default at the bottom of the market
Ordinary mineral glass is hardened silicate glass. It scratches where sapphire does not, but it is cheap, it is more impact-tolerant than sapphire, and it is clear and low-glare without coating. On a watch that costs less than a strap, it is exactly the right choice.
The Casio Duro MDV106 is the case in point: a 44mm 200m quartz diver for under seventy dollars with a mineral crystal. Nobody should want Casio to have fitted sapphire and charged double. Mineral is also what Citizen fits across most of the Promaster and Eco-Drive range even at three hundred dollars, which is the one consistent criticism we make of the brand on the Citizen brand page.
Acrylic: the one that scratches and does not matter
Acrylic is plastic. It picks up swirls and scuffs almost immediately, and it is the only material here where that is genuinely not a problem — because scratches polish out. A dab of plastic polish and a cloth, two minutes, and the crystal is clear again. You cannot do that with sapphire or mineral; damage there is permanent short of replacing the crystal.
It is also the toughest material on this page in the impact sense, and it is the reason vintage-styled watches use it deliberately: acrylic can be formed into the high domed profile that gives old watches their warm, distorted edge, which flat sapphire cannot cheaply replicate. The Timex Q GMT is acrylic for exactly that reason, and it would be a worse-looking watch with a flat crystal.
Resin glass: the twenty-dollar answer
Resin glass is the cheapest option and it turns up on the digital Casios — the Casio F-91W here uses it. It scratches, it is not especially clear, and at this price the watch is a consumable rather than an heirloom. It is the correct material for a watch designed to be replaced rather than serviced, and criticizing it is like criticizing a disposable pen for not being refillable.
How each one fails, which is the real decision
Hardness tells you what scratches. The more useful question is what happens when something goes wrong.
Sapphire survives everything until it chips or shatters, and then it is a crystal replacement — not catastrophic, but a trip to a watchmaker. Mineral and Hardlex accumulate fine scratches gradually and visibly, and can also crack; neither polishes out. Acrylic scratches constantly, polishes out indefinitely, and tends to craze or deform rather than shatter. Resin does whatever it does for a few years and then you buy another watch.
If the watch is going somewhere it can be struck hard — a worksite, a climbing wall — the brittleness of sapphire is a genuine consideration and the shock-resistant resin-cased option is the serious answer. For everyone else, scratch resistance is the thing you will actually notice.
Is sapphire worth paying extra for?
Yes, in two situations and not in a third.
Buy sapphire when the watch is one you plan to keep for years, or when it is going to the beach, the sea or anywhere sandy. That is where the hardness gap stops being theoretical. Buy sapphire also when it is free — which, at the Expedition North and Kamasu end of the market, it effectively now is.
Do not pay a large premium for it on a cheap watch you expect to wear hard and replace. A $65 Casio with mineral glass and a 200m rating is a better object than a $130 version of the same watch with sapphire, and the crystal is not where that money should go. Spend it on water resistance and a screw-down crown instead, which is the spec most buyers under-weight.
Can you tell which crystal a watch has?
Not reliably by eye, which is why the spec table above exists. Manufacturers state it, and where we could not verify it from a manufacturer we print a dash rather than a guess. Two weak tells in person: sapphire feels colder to the touch for a moment longer than glass because it conducts heat better, and a drop of water beads more tightly on an uncoated sapphire surface. Neither is conclusive, and neither is worth doing in a shop — read the specification instead.
