Plexiglass, mineral glass, and sapphire – differences and applications

When replacing the glass of a watch, it is important to choose not only the appropriate diameter, thickness, and profile but also the material.

The most commonly found are:

• plexi glasses – acrylic,
• mineral glasses,
• sapphire glasses.

Each of these materials has different properties. This does not mean that the most expensive material will always be the best solution for every watch.

Plexi Glass – What Is It?

Plexi is a transparent plastic material commonly used in older watches, vintage watches, pocket watches, and designs that require a characteristic glass profile.

It is a softer material than mineral and sapphire glass, which is why fine scratches can relatively easily occur on it.

At the same time, plexi has a significant advantage – many superficial scratches can be polished out, which means that replacing the entire glass is not always necessary.

Plexi is also more flexible than mineral or sapphire glass.

See plexi glasses for watches

When to Use Plexi Glass?

Plexi is particularly a good choice if the watch was originally equipped with this type of glass.

This applies especially to:

• older mechanical watches,
• vintage watches,
• pocket watches,
• models requiring a high or unusual glass profile,
• watches where plexi is a structural element of the case.

One should not automatically replace plexi with mineral or sapphire glass just because these materials are harder. The case design may be specifically adapted for plastic glass.

Mineral Glass – The Most Popular Choice

Mineral glass is very commonly used in modern watches. It is harder than plexi and more resistant to everyday scratches.

One of its greatest advantages is the vast number of available variants.

Mineral glasses come in:

• flat,
• convex – spherical,
• lens-shaped,
• of various thicknesses,
• with a very large number of diameters,
• with additional elements, e.g., a magnifying glass.

See mineral glasses for watches

Can Mineral Glass Be Polished Like Plexi?

Mineral glass should not be treated the same way as plexi. Typical fine scratches on plexi can be relatively easily removed by polishing, while removing scratches from mineral glass is significantly more difficult.

In practice, with heavily scratched or damaged mineral glass, replacing it is often more justified.

Sapphire Glass – How Does It Differ from Mineral Glass?

Sapphire glass is made from synthetic sapphire and is characterized by very high hardness and scratch resistance.

This is its most important advantage over plexi and typical mineral glass.

However, this does not mean that sapphire glass is indestructible. It can still be damaged by a sufficiently strong impact.

See sapphire glasses for watches

Is Sapphire Glass Always Better than Mineral Glass?

Not always. If we evaluate solely the surface scratch resistance, sapphire glass has a significant advantage.

However, when repairing a watch, one must also consider:

• the case design,
• the diameter of the glass,
• the thickness,
• the profile,
• the mounting method,
• the gasket,
• the original design of the watch.

Not every mineral glass can simply be replaced with a sapphire one of similar diameter.

Plexi, Mineral, and Sapphire – The Key Differences

Feature	Plexi	Mineral	Sapphire
Scratch Resistance	Low	Medium	Very High
Ability to Remove Fine Scratches	Often Yes	Significantly More Difficult	Very Difficult
Flexibility	Greatest	Less	Less
Availability of Different Profiles	Very High	Very High	Less
Surface Resistance to Everyday Scratches	Least	Greater	Greatest
Price	Usually Low	Usually Moderate	Usually Highest

Can Plexi Be Replaced with Mineral Glass?

Sometimes this is possible, but it cannot be assumed as a general rule.

Plexi and mineral glass can be mounted in completely different ways. The differences also concern the profile, edge thickness, and how the glass interacts with the case.

If the watch originally had plexi, the safest solution is to choose the appropriate plexi glass unless the case design has been checked and allows for a different mounting method.

Can Mineral Glass Be Replaced with Sapphire?

In some watches, this is possible, but simply finding sapphire glass of the same diameter does not mean it will be a proper replacement.

It is essential to compare primarily:

• the diameter,
• the thickness,
• the edge profile,
• the total height,
• the mounting method,
• the glass gasket.

Are 30.4 mm Mineral and 30.4 mm Sapphire Glass Interchangeable?

This cannot be determined based solely on diameter.

Both may have a diameter of 30.4 mm, but for example:

• one may be 1.0 mm thick, while the other is 1.5 mm,
• one may be flat, while the other is convex,
• they may have differently finished edges,
• they may require different mounting methods.

Diameter is therefore just one of the parameters.

Material Is Not the Same as Glass Profile

This is a very important distinction.

Mineral, sapphire, and plexi primarily define the material.

Whereas the terms:

• flat,
• spherical – convex,
• lens-shaped

describe the shape and profile.

For example, we can have:

• flat mineral,
• spherical mineral,
• lens-shaped mineral,
• flat sapphire,
• convex plexi.

See: flat, spherical, and lens-shaped glass – differences

How to Recognize What Old Glass Is Made Of?

If the material is unknown, it is not worth determining it solely based on appearance.

An experienced watchmaker can recognize many glasses based on construction, material behavior, mounting method, and other features, but when selecting parts, it is best to also use information about the specific watch.

How to Choose New Glass?

Regardless of the material, it is necessary to check:

1. the type of old glass,
2. the profile – flat, spherical, lens-shaped, or other,
3. the diameter after removing the glass from the case,
4. the thickness,
5. in the case of lens-shaped glass – the thickness at the edge and in the center,
6. the mounting method,
7. the type and size of the gasket, if present.

For accurate measurement, a caliper should be used. One should not choose glass based on measurements taken from above while the glass is still in the watch.

See: how to properly choose and measure glass for a watch

What to Choose – Plexi, Mineral, or Sapphire?

If we are performing a standard repair, a good starting point is to replicate the factory solution.

One should not choose glass solely based on the principle that "sapphire is the best." The right glass is one that fits the design of the specific watch in terms of material, dimensions, profile, and mounting method.

Glasses for Watches at Kurant Wholesale

The watchmaking wholesale Kurant has in stock over 80,000 pieces of watch glasses in a very large number of diameters, thicknesses, and profiles.

Among the offerings are:

• plexi glasses,
• mineral glasses,
• sapphire glasses.

Thanks to the large number of available dimensions, one can choose glass for both popular modern watches and many older or unusual designs.