The second very important part of every watch is the crystal that is protecting watch dial. And they vary based on durability and practicality. I will start with weakest and finish with strongest…
I’ve added rough estimated hardness values based on Vickers scale for each crystal type per user request.
Acrylic crystal (>20 Vickers)
Acrylic crystal is basically a fancy name for clear plastic glass (plexiglass if you want). It is by far the cheapest and lightest and nearly impossible to shatter. But is very weak against scratches. Every metal object will leave a mark on it, so be very careful with such types of crystals. Acrylic crystals are very common on dive watches because you can’t shatter them and potentially destroy the whole watch if the water comes in. If you damage it, it’s cheaper to replace the crystal than to buy a new watch because the last one drowned…
On the other hand, acrylic crystal has one unique capability. It can be polished to remove tiny scratches. It can only be done few times, but is an option you can take with acrylic crystals.
Mineral crystal (>350 Vickers)
Mineral crystal is again a fancy name for ordinary glass. Quite scratch and shatter resistant, but will sooner or later get few tiny scratches. However you have to hit it against something really hard to leave a bigger scratch or to shatter it. Unfortunately there is no way to polish the mineral glass like you can acrylic glass, so every scratch on it is a permanent one.
Hardlex crystal (>650 Vickers)
Hardlex is a special proprietary crystal designed by Seiko to fill the gap between mineral glass and sapphire glass. It’s a special hardened glass that is much more scratch resistant than mineral glass and a lot more shatter resistant than sapphire crystal. Hardlex can be found on pretty much every single Seiko watch that is not using acrylic or sapphire crystal. Like mineral crystal, every scratch on it, is a permanent one.
Sapphlex crystal (~2000 Vickers)
While not being used anymore, it was an interesting concept developed and used by Seiko watches in the 90’s. It’s a laminated mineral/sapphire crystal glass that had mineral crystal underneath, covered by a layer of sapphire crystal. In theory, it should be very scratch and shatter resistant at the same time. However, the main problem with certain watches was with delamination, a process where mineral and sapphire crystal layers separated and that’s not cool. Those fortunate enough to have a Sapphlex on their watch that didn’t delaminate are however very happy because it was truly as scratch resistant as 100% sapphire crystal.
FlameFusion crystal (~2000 Vickers)
This is a special type of crystal used by Invicta watches. It is very similar to Sapphlex, but is not a laminated glass, it is in fact a fused mineral and sapphire crystal into a single material that cannot delaminate like Sapphlex did in some cases. That’s why they call it FlameFusion because it’s a process where materials are fused together at very high temperatures. FlameFusion crystal is very scratch resistant and also very shatter resistant. Still not as scratch resistant as pure sapphire, but very close.
Krysterna crystal (~2000 Vickers)
Krysterna crystal is a proprietary synthetic crystal used by top range eye wear like high end sunglasses, but also adopted by Stührling Original for their watches. It is advertised to be as scratch resistant as sapphire, but with higher impact resistance, making it less prone to shattering on impact. Technology in creating such crystals is very similar to FlameFusion (Verneuil process).
Sapphire crystal (>2000 Vickers)
Sapphire crystal, the holy grail of crystals. The most scratch resistant material used for watch faces. It’s a synthetic sapphire, a lot cheaper than natural one, but with pretty much exactly the same physical properties. And can be mass produced. If you see that watch has “sapphire crystal” listed under specifications, you’re looking at a very good crystal that is nearly impossible to scratch where mineral glass or Hardlex would already scratch. Some say sapphire crystal is brittle and can be shattered rather easily, I have yet to see a shattered watch crystal. However I’ve seen plenty of scratched crystals, so for me, scratch resistance is far more important than shatter resistance.
Carbon Crystal/Diamond (>10.000 Vickers)
Actually I lied. Sapphire crystal is not the strongest material used for watches. As we know it, diamond is the strongest crystal material (if we exclude hyper diamonds). Carbon Crystal is a brand used by Cartier and basically means synthetic diamond. Cartier used it to manufacture lubrication and adjustment free escapement mechanism that basically cannot wear out like metal escapements always will. However, I’m not aware of them actually using Carbon Crystal to manufacture synthetic crystal to protect the dial of the watch. In fact, I’ve never heard of any watch to use synthetic diamond. If there is one, it is either very rare watch or just a concept that never went into general production. Because pretty much all highest end watches still use sapphire crystal, even those that cost so much, you’d expect a synthetic diamond to be used for its crystal.
Unless there are any specific reasons not to use sapphire crystal, I recommend everyone to decide for the sapphire crystal. It’s superior in scratch resistance and you have to be really really clumsy to shatter it. Watches with sapphire crystal do cost more, but they are well worth it, trust me. Because it’s nothing worse than knowing you have a scratch on your watch glass and you can’t do anything to get rid of it (except replacing the whole crystal which is quite costly). No such problems with sapphire. So, think about it before you decide not to buy a watch with sapphire crystal just because it costs a bit more. Though, I do know that not all watches come with sapphire, so if you like the design of the watch, but only comes with mineral crystal, well, that is a tradeoff that you have to take in such cases. I’ve bought a Casio with acrylic crystal some time ago because I really liked the looks of it. But I pick sapphire whenever it is possible in all other cases.