Watch crystal types

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.

14 thoughts on “Watch crystal types

  1. I would like to have seen some hardness numbers quoted just to complete the comparison picture. Otherwise this is a reasonably complete summary with little opinion bias.


    1. I can update the article. Though hardness alone doesn’t really tell how good a crystal is. Acrylic hardly has any hardness, but can be superior for certain situations because it is flexible material and thus shatter proof. But in most cases, I’d prefer sapphire. I have yet to see one shattered, but I’ve seen tons of crystals being scratches because they are acrylic or mineral…


    2. I’ve added some hardness numbers to each crystal type based on Vickers hardness scale. I don’t have further details on crystals though, I know that for metals, it is a linear scale so doubling the value means double the hardness. I guess the same applies to minerals?


      1. It is much harder than you might expect to obtain relevant & useful hardness info as I have just discovered. Beware of any hardness given using the Moh scale because its generally misleading.


      2. Moh scale is generally only used for crystals/minerals, Vickers is used for pretty much everything from plastics, metals, crystals etc. Plus, it’s a lot more accurate. Tough be aware that my numbers are estimations as well since values vary depending on actual materials used, their quality, thickness etc and these parameters tend to be very different from watch to watch so even watches with acrylic crystal might not behave the same and one might scratch faster than other.

        And there is also design factor. For example, vintage watch styles usually have domed crystals that are super exposed and if they aren’t sapphire, they’ll scratch super fast. But if you use same weak crystal and recess it within the watch bezel, it can withstand lots of torture. Orient M-Force is designed this way and pretty much all G-Shocks. G-Shocks only use mineral crystal and they rarely get scratched on the crystal, because the bezel takes the beating before crystal does…


  2. Flame fusion is garbage. In reality it is the Verneuil process which was invented at the turn of the century, the 20th century, 1902 or so. Nothing more than a marketing ploy and a trademarked name for something over 100 year old. It delaminates just as easily as other fake sapphire dials.


    1. I wouldn’t say something is garbage just because the process is 100 years old. There are many things that base on 100 years old methiods and are still used today. It just means we haven’t figured out a better method or it’s just so good we still use it. Also, FlameFusion is Invicta’s brand name, not a name of the manufacturing process. They might got name inspiration from this old manufacturing process but doesn’t necessarely means it’s that process used to create crystals..


      1. What I am saying is “flame fusion” is garbage. It also does delaminate and there are many complaints about it happening. Furthermore is it really “flame Fusion” or another process licensed from another company.

        Flame Fusion Crystal is the name they trademarked. It is a marketing ploy to reduce costs and increase profits. Considering that most of the buyers of Invicta watches would not know the difference between a turnip and a cow, they get all worked up with fancy sounding names and oversized POS.

        I agree that Sapphire is the best choice for a crystal. It may be a bit more expensive but given that it is almost impossible to scratch well worth the price. I also agree with your observation in regards to shattering of the Sapphire crystal. I have been collecting watches over 50 years and to date have never shattered one.


  3. I recently picked up a Brandt & Hoffman. On their site it says they have an “Anti-Reflective and Scratch-Resistant Sapphire Coated Crystal”. Somehow I got it into my head that this means that they have sapphire crystals. Am I wrong? Is it somehow just sapphire coated?


    1. The wording is a bit unfortunat to be quite honest.

      “Anti-Reflective and Scratch-Resistant Sapphire Coated Crystal” can actually mean two things:

      – Anti-reflective coated scratch-resistent sapphire crystal (sapphire tends to glare more, that’s why they apply special coating on the inside part of the sapphire, some even do it on both sides)

      – A sapphlex style mineral crystal with sapphire top layer and anti-reflective coating.

      Frankly, I think they just worded it a bit weirdly and they are just using “usual” sapphire crystal with anti-reflective coating. Sapphire is really cheap these days, I can’t see any reason why anyone would bother with sapphlex like crystals…


  4. Our company is beginning the production of cvd grown diamond watch crystals. We are currently looking for watch manufacturers who would want to participate in early stage development. We can produce monocrystal and polycrystal diamond plates over 2mm thick.


    1. That’s interesting. Do you by any chance have a website of your company? I think you’ll need that to make some sort of general exposure to watch makers. I’d suggest getting in touch with watch makers like Christopher Ward and Steinhart. These are smaller companies that are more willing to experiment than big ones like Rolex. Certainly check out WatchUSeek forums, it has many watchmakers participating there, maybe you could find relevant contacts there.


  5. Please let me know how can I make the difference between Mineral Crystal (ordinary glass) and Hardlex Crystal glass


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