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Sapphire, mineral or acrylic?
The crystal is the part of a watch you look through every day and the part most likely to be damaged. Pick one below to see what will and will not scratch it.
The Mohs scale, and why 9 versus 5.5 matters so much
Mohs is a relative scale of scratch hardness: anything higher on it scratches anything lower. Sapphire sits at 9, mineral glass around 5.5, acrylic at about 3. The critical number in between is quartz, at 7, because ordinary sand and household dust are largely quartz.
That single fact explains most crystal damage in the real world. Sand scratches mineral glass and acrylic easily because they sit below 7, and cannot touch sapphire because it sits above. A beach holiday will visibly age a mineral crystal and leave a sapphire untouched.
The scale is also not linear. The gap between 9 and 10 is far larger in absolute terms than the gap between 3 and 4, which is why sapphire is so dramatically more scratch resistant than its number suggests.
Hardness and toughness are opposites
The common assumption is that sapphire is simply better. It is harder, but hardness and toughness are different properties, and improving one usually costs you the other.
Sapphire is brittle. It resists deformation so completely that when the force gets high enough it fractures rather than dents. A corner knock against a door frame that would leave a scuff on acrylic can put a crack right through a sapphire crystal, and a cracked crystal means replacement rather than repair.
Acrylic does the opposite. It is soft enough to mark if you look at it wrong, but it absorbs impact by flexing, and every one of those marks polishes out with a mild abrasive in a few minutes. Over ten years an acrylic crystal that has been occasionally polished can look better than a mineral one that has never been touched.
Which to choose
For a watch you wear daily and want to look new indefinitely, sapphire, and accept that a hard knock is a replacement rather than a polish. This is the right answer for most buyers, and it is now standard well below the luxury tier.
For a vintage piece, acrylic is often correct and worth keeping. Replacing the acrylic on a 1960s watch with sapphire changes how the dial reads, because the slight warmth and distortion of the original is part of how the watch was designed to look. Collectors generally regard it as a downgrade.
Mineral glass is the one to be wary of. It combines the scratch vulnerability of a softer material with the unrepairability of a harder one, so damage accumulates permanently. If a watch you like has mineral glass, budget for eventually replacing the crystal or accept that it will look progressively worse.
Common questions
Is sapphire crystal worth it?
For a daily wear watch, yes. Sapphire sits at 9 on the Mohs scale, above the quartz in sand and dust at 7, so ordinary life will not mark it. The trade-off is that it is brittle and cracks rather than scratches under a hard impact, and a cracked sapphire has to be replaced.
Can you scratch sapphire crystal?
Only with something at or above 9 on the Mohs scale, which in practice means diamond, silicon carbide such as some sandpapers, and other sapphire. Steel, keys, sand, concrete and glass will not touch it.
Why does the Omega Speedmaster still use acrylic?
Because it does not shatter. NASA specified hesalite for the Moon missions since a cracked crystal in a spacecraft cabin means floating shards. Omega kept it on the Moonwatch for authenticity, and it has the practical benefit that scratches polish out at home.
Can mineral glass scratches be polished out?
Not really. Light hazing can be improved with a cerium oxide compound, but any scratch you can catch with a fingernail is permanent. Mineral crystals are usually replaced rather than repaired, which is inexpensive but requires a watchmaker.
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