Different Types Of Watch Glass – A Guide To Watch Crystals

Simply put, the primary purpose of a watch crystal is to protect the dial and allow you to read the time.

But that’s not all. There’s a lot more to know about watch glass.

For the average watch enthusiast, it can be pretty confusing to know the different types of watch glasses and what they all stand for.

If you are unsure what types of watch glass there are and how they differ, you are in the right place.

In this article, I will explain to you:

  • What all types of watch crystals are there?
  • What are the advantages and disadvantages of the different kinds of watch glass?
  • Which watch glass is the best for you?
  • How to recognise the different kinds of watch glasses?

Read on to learn which crystal is best for you the next time you want to buy a watch.

Three most common used watch crystals

Types of Watch Glass

The three most commonly used types of watch glass are acrylic crystal, mineral crystal, and sapphire crystal.

Each of them has different properties suitable for other conditions of use, but they also differ in price.

Of course, each of them has its advantages and disadvantages.

Acrylic Glass / Acrylic Crystal

Acrylic crystal is the cheapest type of crystal. Until the 1980s, acrylic glass was commonly used.

This type of glass is also known as Plexiglas or Plexi for short.

Since this type of glass is actually plastic, it is translucent, flexible, and extremely lightweight.

While it is difficult to break, it is easy to scratch.

On the Mohs hardness scale, acrylic plastics usually have a rating of about 3 or 4 out of 10.

This means that most hard objects are likely to leave a mark or scratch when they come into contact with an acrylic crystal.

Fortunately, the scratches can be easily polished out.

For the production of inexpensive watches, watchmakers usually use acrylic glass.

Since it is made of plastic, acrylic crystals can be moulded easily into various shapes, unlike sapphire and mineral crystals.

Interestingly, acrylic crystal is often used in rugged timepieces because of its flexibility, which surprisingly makes it shock resistant.

Another interesting feature of acrylic glass is that it is resistant to fingerprints.

However, due to micro-scratches, it becomes cloudy after some time, which affects the visibility of the dial.

ProsCons
LightweightExtremely easy to scratch
Easy to polishThe glass becomes “cloudy” over time
Cheap (if you need to replace it)It has a cheap look
Shock resistant
Fingerprints resistant
Pros and cons of plexiglass on watches
This Longines watch uses an acrylic crystal

Mineral Glass / Mineral Crystal

The most common form of watch glass on the market today is mineral crystal.

This glass is often used for mid-range watches, as it is an inexpensive glass option.

Mineral glass is harder than acrylic but softer than sapphire glass.

It is almost the same glass as that used for windows.

It is made of silica and has a Mohs hardness rating of 5 out of 10.

Mineral glass is more scratch resistant than acrylic glass, but scratches on the surface are much more difficult to polish.

Also, mineral glass can crack or break at extremely high or low temperatures.

A Fossil watch using a mineral crystal

A special type of mineral glass developed by Seiko is Hardlex crystal.

Hardlex is made by heating and chemically treating ordinary glass.

These modifications make the mineral more scratch and break-resistant.

Hardlex crystal is mostly used in entry-level Seiko watches.

Seiko has been using Hardlex since 1970 and registered the name as a trademark in 1980.

In my opinion, in reality, Hardlex does not perform notably better than ordinary mineral glass.

The transparency of the mineral glass is better than that of sapphire, so it is not necessary to use anti-reflective coatings.

Overall, mineral glass is in the middle between acrylic and sapphire glass, both in terms of features and price.

A Seiko watch using the Hardlex crystal
ProsCons
More scratch resistant than acrylic glassScratches are difficult to polish
Pretty impact resistant
Less reflective than sapphire glass
Relatively cheap (if you need to replace the glass)
Pros and cons of the mineral watch glass

Sapphire Glass / Sapphire Crystal

Sapphire crystal is the most desirable and expensive type of watch glass.

Synthetic sapphire is used to make sapphire crystal.

Synthetic sapphire crystal is not actually glass at all.

It is made by crystallizing pure aluminium oxide at very high temperatures and is incredibly hard.

Its exceptional hardness makes it the perfect choice for high-end timepieces.

With a hardness of 9 out of 10 on the Mohs hardness scale, sapphire is the second hardest substance on earth.

It is believed that only diamonds and other sapphires can scratch sapphires.

Thanks to its scratch resistance, the sapphire crystal still looks new even after years of use.

Although sapphire crystal has an extremely high hardness, it is not very impact resistant.

If you drop your watch on a hard surface or accidentally knock it against a hard object, the glass may break easily.

Sapphire crystals are very reflective.

It reflects light more than mineral crystals.

A quick solution to this problem could be an anti-reflective (AR) coating.

The reflection is significantly reduced by these coatings.

Most luxury watches put their AR coating under the sapphire crystal.

This prevents scratches on the surface of the coating.

Sapphire crystal has an impressive clarity that not only makes it look flawless but also makes the underlying dial appear sharper, but it is expensive to produce and difficult to make in complicated shapes like domed glass.

What makes sapphire crystal expensive to produce are the costly machines used to cut and polish this material.

One of the more recent innovations in the world of watches is sapphire-coated mineral glass.

This is actually ordinary mineral glass with a sapphire coating on the top.

As with the Hardlex crystal, I do not have much faith in this option of mineral glass reinforcement either.

So if you want the best scratch-resistant watch glass, you should choose a sapphire crystal.

ProsCons
Highly scratch resistantHigh price
Luxury lookLess impact resistant than mineral and acrylic glass
Pros and cons of the sapphire watch glass
An Omega watch using the sapphire crystal

How To Recognise The Different Types Of Watch Glass

When you buy a watch in official stores, you will always receive accurate information about what kind of glass is on the watch you want.

However, if you buy a watch on the used market or own a watch that doesn’t come with the specs, there are a few ways to find out what kind of glass your watch has.

Since acrylic glass is made of plastic, it is entirely different from mineral or sapphire glass.

You can feel the difference when you put it on your cheek.

Acrylic glass is usually warm, while mineral glass or sapphire crystal is cold.

However, it is much more challenging to distinguish mineral glass from sapphire, but there are a few tricks for that.

You can do a test with a drop of water on the glass.

If the water maintains its drop shape, it is most likely a sapphire crystal.

On the mineral glass, on the other hand, the water will flow and spread on the surface of the glass.

There is also a sound test by tapping the glass with your fingernail.

If the sound is firm and low, it is most likely sapphire crystal.

If the sound is high, it is most likely mineral glass.

An Omega watch using acrylic crystal (You can spot the scratches on the plexiglass)

Conclusion

You can choose a watch with a suitable glass depending on your preferences and budget.

Sapphire crystal is definitely the best, but that doesn’t mean it’s always the best choice.

I think the mineral glass is the least interesting option compared to sapphire and acrylic glass.

Neither sapphire nor acrylic glass can be classified as the “ideal” material overall for watch crystals; each has its own advantages and limitations.

Sapphire crystal gives a watch a more modern and elegant look and feel, and thanks to its superior scratch resistance, the watch face will remain crystal clear and flawless for many years.

On the other hand, many collectors prefer acrylic crystals because of the warm retro look they give a watch.

If you ask me, my choice for modern watches is sapphire crystal, while I prefer acrylic crystal for vintage watches or retro-look watches.