Sapphire glass in watches – properties and structure of the material

A new shipment of sapphire watch crystals has arrived in our offer. This is a good opportunity to explain what sapphire used in watchmaking actually is and why its properties differ so much from typical mineral glass.

See available sapphire watch crystals at Hurtownia Kurant

What is sapphire glass?

The term "sapphire glass" is commonly used in watchmaking, but from a material perspective, sapphire is not classical glass.

The sapphire used for watch crystals is a transparent, monocrystalline aluminum oxide – Al₂O₃. This is the same basic crystalline material found in natural sapphire, but for technical applications, it is produced industrially under controlled conditions.

This allows for a clear material with consistent properties, which is then cut, polished, and finished to the required dimensions.

Sapphire glass is not mineral glass with a coating

This is an important distinction. True sapphire glass is not just ordinary mineral glass coated with a thin layer of sapphire.

In the case of full sapphire glass, the entire element is made from sapphire material.

Therefore, its properties arise from the material's structure itself, not just from the applied surface layer.

Why is sapphire so scratch-resistant?

The most important property of sapphire in a watch is its very high hardness.

Sapphire achieves a value of about 9 on the Mohs scale. It is this high surface hardness that makes it significantly harder to scratch during everyday use compared to typical mineral glass.

Contact with objects that could leave scratches on ordinary watch glass rarely causes permanent damage to the surface of sapphire.

Can sapphire glass be scratched at all?

Yes, it can. The term "scratch-resistant" does not mean "impossible to scratch".

Sapphire is a very hard material, but there are materials and abrasives with sufficient hardness to damage its surface.

In practice, its resistance to typical scratches that occur during watch use is, however, significantly greater than that of standard mineral glass.

Hardness vs. impact resistance – they are not the same

It is important to distinguish between two concepts:

• hardness – surface resistance to scratching and abrasion,
• mechanical impact resistance – the ability of an element to withstand sudden loads without breaking or chipping.

Sapphire is extremely hard, but that does not mean that sapphire glass is indestructible.

A strong impact, especially on the edge of the glass, can still lead to chipping or cracking.

Therefore, the term "sapphire" should primarily be understood as an indication of very high surface scratch resistance, not a guarantee of resistance to every impact.

Why is sapphire well-suited for watch glass?

The material used for watch glass must combine several properties. It should be:

• transparent,
• abrasion-resistant,
• scratch-resistant,
• stable during long-term use,
• capable of very precise grinding and polishing.

Sapphire meets these requirements, which is why it is used not only in watchmaking but also in many precise technical and optical applications.

Sapphire glass vs. mineral glass – the main difference

The fundamental difference lies at the very material level.

Feature	Sapphire Glass	Mineral Glass
Material type	Monocrystalline aluminum oxide Al₂O₃	Classic glass with a mineral composition
Scratch resistance	Very high	Lower
Surface hardness	Very high	Significantly lower
Processing	More difficult due to high hardness	Easier
Cost	Usually higher	Usually lower

This does not mean that mineral glass is a bad solution. It is used in a vast number of watches and performs very well in many applications.

Sapphire is chosen primarily when we particularly care about keeping the glass surface scratch-free during long-term use.

Why is sapphire glass more expensive?

The price difference arises, among other things, from the manufacturing and processing of the material.

Sapphire must first be obtained in the form of an appropriate crystal, and then precisely cut, ground, and polished.

Its very high hardness, which is an advantage during later watch use, also complicates the processing during production.

Is sapphire glass always perfectly colorless?

Technical sapphire used as a transparent optical element is characterized by very good clarity.

However, the final appearance of the glass may also depend on the way it is polished and any additional coatings.

What about the anti-reflective coating?

An anti-reflective coating and sapphire material are two different things.

The glass can be made of sapphire and additionally have a layer that reduces light reflections. However, the coating itself is not what makes the glass sapphire.

It is also worth remembering that the properties of the additional surface coating may differ from those of the sapphire beneath it.

Can mineral glass be replaced with sapphire glass?

In many cases, this is possible, but the decision should not be made solely based on the material.

The new glass must primarily have the correct:

• diameter,
• thickness,
• shape,
• method of mounting in the case.

If the original mineral glass has an unusual profile, shape, or mounting method, flat sapphire glass of similar diameter may not be a suitable replacement.

Why is precise measurement so important?

When selecting glass for a watch, a difference of hundredths of a millimeter can be significant.

The most accurate measurement is made after removing the old glass from the case. Measuring the opening visible from the top of the watch may not correspond to the actual diameter of the glass in the socket.

It is best to use a precise caliper for measurement.

Can the thickness of the glass be changed?

The thickness should be chosen considering the construction of the specific watch.

Changing the thickness can affect, among other things:

• the method of mounting the glass,
• its position relative to the case,
• the distance from the hands,
• the appearance of the watch,
• the interaction of the glass with the gasket.

Therefore, it should not be assumed that any glass with the correct diameter will be suitable.

For whom is sapphire glass a good choice?

Sapphire glass is worth considering especially when:

• the watch is used intensively,
• it is important to keep the surface free of minor scratches,
• the watch is being refurbished to a higher class,
• sapphire glass was originally used,
• the case design allows for a suitable replacement.

How to care for sapphire glass?

High scratch resistance does not mean that the watch does not require normal protection.

It is advisable to avoid strong impacts on the edge of the glass and contact with abrasive materials. Usually, a soft, clean cloth is sufficient for cleaning the surface.

Sapphire watch crystals at Hurtownia Kurant

A new shipment of flat sapphire watch crystals in various diameters and thicknesses has arrived in our offer.

Before purchasing, be sure to check the required dimensions of the old glass and the method of its installation in the case.

See available sapphire watch crystals and choose the right size