Sapphire-coated mineral glass: what exactly is it?

What is sapphire-coated mineral glass?

Many watch enthusiasts are now familiar withsapphire crystal. It is extremely hard, highly scratch-resistant and is widely used in high-quality watches. That’s why you’ll also find a separate selection of watches with sapphire crystal at WatchXL.

But what does it mean when a watch’s specifications mention ‘mineral glass with a sapphire coating ’ or ‘sapphire-coated mineral crystal’? The key point is simple: this is not solid sapphire crystal.

The base of the watch crystal is made of toughened mineral glass. A very thin, hard and transparent coating is then applied to the surface. This gives the mineral glass extra protection against scratches.

The coating is extremely thin compared to the mineral glass itself. Therefore, no thin sheet of sapphire is placed on top of the mineral glass. It is a protective surface layer.

Aston Martin watch with mineral glass featuring a sapphire coating.

How is a sapphire coating applied to mineral glass?

There are various techniques for applying hard, transparent coatings. A well-known method for applying thin layers of material is Physical Vapour Deposition, usually abbreviated to PVD. You may also be familiar with this term from coloured watch cases, where a thin layer is applied to the base material.

During the coating process, the mineral glass is first carefully cleaned. A very thin layer of material is then deposited onto the surface in a controlled environment. Depending on the manufacturer, the process used, the composition and the thickness of the coating may vary.

Aluminium oxide, with the chemical formula Al₂O₃, is of interest in this context because synthetic sapphire also consists of aluminium oxide. However, this does not mean that mineral glass with a thin layer of aluminium oxide is the same material as full sapphire glass.

In the case of full sapphire glass, the glass itself consists of synthetic sapphire crystal. In the case of mineral glass with a sapphire coating, the base material remains ordinary mineral glass.

TW Steel with mineral glass and a sapphire coating.

A sapphire coating is not sapphire glass

This often leads to confusion in practice. If a watch is described as having ‘sapphire-coated mineral crystal’, this does not mean that the watch has a full sapphire crystal.

The difference is not always immediately visible to the naked eye, but technically speaking, these are distinctly different types of watch glass. Anyone who prioritises maximum scratch resistance would therefore be best advised to look specifically for watches that feature full sapphire glass.

Why do manufacturers use mineral glass with a sapphire coating?

Why wouldn’t a watch brand always opt for sapphire crystal straight away? Production costs and market positioning play a significant role in this.

Mineral glass is relatively inexpensive to produce and offers good properties for everyday use. Full sapphire crystal is considerably harder and much more scratch-resistant, but the material is also more expensive to produce and process.

Mineral glass with a sapphire coating therefore offers an interesting compromise. You retain mineral glass as the base, whilst a hard top layer ensures the surface receives extra protection.

For watches where a manufacturer wishes to offer greater scratch resistance without immediately resorting to full sapphire crystal, this construction can therefore be a logical choice.

What happens if the sapphire coating is damaged?

The protective coating is thin. If this layer becomes damaged or wears away in places due to intensive use, the softer mineral glass underneath offers less protection against scratches in those areas.

This is an important difference compared to full sapphire crystal. With the latter, it is not just the surface that is hard; the entire watch crystal is made of the scratch-resistant material.

Sapphire-coated mineral glass is therefore better protected against scratches than standard mineral glass, but it should not be equated with solid sapphire crystal.

Do you want to prevent unnecessary scratches? In our Watch News, you can also read practical tips on how to keep your watch crystal scratch-free.

Not every sapphire coating is the same

Manufacturers may use different coating processes, materials and layer thicknesses. Consequently, one sapphire coating does not necessarily have to have exactly the same properties as another.

The description ‘sapphire-coated mineral crystal ’ alone is therefore insufficient to compare two watches in detail in terms of the quality of their coatings.

The term mainly tells you that the watch features mineral glass with an extra-hard protective layer. For precise technical details, you must rely on the information provided by the manufacturer regarding the coating used.

What does WatchXL include in a watch’s specifications?

At WatchXL, we believe it is important that you know exactly what you are buying based on the product specifications.

If a watch has mineral glass with a sapphire coating, we believe this should be clearly stated in the specifications. Using only the term ‘sapphire glass’ would suggest that the entire watch glass is made of synthetic sapphire.

We therefore make a clear distinction between mineral glass, mineral glass with a sapphire coating and full sapphire glass.

Would you like to know more about these differences? Then read our in-depth article ‘Mineral glass or sapphire glass: which is best for your watch?’

Is sapphire-coated mineral glass any good?

Think of it as a middle ground between mineral glass and sapphire crystal. Standard mineral glass forms the base and is perfectly suitable for normal use. The extra coating helps to protect the surface better against scratches.

However, if you want the highest scratch resistance of these three options, full sapphire crystal is the better choice. In this case, the entire crystal is made of the hard material, not just the outer layer.

That doesn’t mean a watch with a sapphire coating is a bad choice. On the contrary. For many watches worn on a daily basis, it offers an attractive combination of protection, ease of use and price.

Are you looking for a new watch and is scratch resistance a key factor for you? Then you can browse the range of watches with sapphire crystal at WatchXL straight away.

Want to read more about sapphire crystal and watch glass?

Would you like to know more before choosing a watch? Then read ‘Mineral glass or sapphire crystal: which is best for your watch?’. There, we explain the key differences between the types of glass in more detail.

Are you particularly curious as to why full sapphire crystal is so popular amongst watch enthusiasts? Then take a look at ‘The benefits of a watch with sapphire crystal’ or read why a watch with sapphire crystal is a smart choice.

Would you particularly like to know how to prevent damage? In ‘Keeping your watch glass scratch-free: here’s how’, you’ll find practical tips on keeping your watch glass in good condition for as long as possible.