Sapphire Crystal

June 5, 2022By Bonetto Cinturini

When we find ourselves looking at a watch with sapphire crystal glass, we almost certainly have a prestigious timepiece before us. It is certainly an optimal choice as far as design is concerned, yet there are other alternatives that are just as valid. It is an optimal material strength-wise, but there are other types of toughened mineral glass that are just as good.

So, can sapphire glass be considered an essential characteristic for watches? Let’s see what sapphire glass offers more than its competitors, mineral glass or plexiglass, can.

Sapphire glass: colour and characteristics of the mineral 

There are many reasons why this material is greatly loved by watch enthusiasts. Firstly, for its beauty and purity, but we must immediately clear up a point.

Sapphire, which comes in different colours, mainly consists of aluminium oxide. In the same way that we associate rubies with the colour red and white with diamonds, it is common to think of sapphire as blue. But we have to make a specification: pure sapphire is not blue. Sapphire that does not have any impurity is 100% corundum, so completely transparent. When sapphires are blue, it is because they contain rutile or hematite. Sapphire, in fact, is not just transparent: it is ultra-transparent. Because of its excessive transparency, an anti-reflection coating is added on both sides to prevent the glass from reflecting and making it difficult to tell the time.

After having established the true colour of this crystal, its incredible strength is the next topic on the list. Sapphire glass is practically indestructible, with a resistance of 9 on the Mohs scale. The Mohs scale is the main scale that assesses the hardness of a material, and it goes from 1 to 10. With a hardness of 9, sapphire is very resistant, and can be engraved and cut only by a material with a scale hardness of 10, in other words a diamond.

Sapphire glass resists knocks, meteorological conditions and water pressure. A good underwater watch for diving will have sapphire glass: resistant to the knocks that it can receive while under the water, and above all resistant to the pressure that can be created.

Thanks to these superlative characteristics sapphire glass is not only used with watches, but also to make smartphone cameras, unbreakable glass and in barcode readers in shops, all materials that have to stay “smooth” and scratch-free.

Types of glass: why prefer sapphire glass

In general, three types of glass are used to make watch crystals. The choice of these materials is dictated by different reasons, but price has the most bearing. There is, in fact, a big difference between the price of a simple plexiglass crystal and one made of sapphire glass, which remains the most prestigious of the three. Let’s see the differences between the three categories, going from the least to the most prestigious:

Acrylic crystal is the least classy material of the three, and it is also the cheapest. It is generally used for low level models, but also for sports watches because in spite of its price and quality, it does have some advantages: it is extremely resistant to pressure and knocks. This is why brands like Rolex and Omega have chosen this material for some of their sports models. This type of glass scratches more easily, but does not shatter if it breaks, a great advantage and something that cannot be said about the other two types of glass.

Mineral glass is treated to improve its resistance to knocks and scratches. It is treated chemically, or heated to make it stronger, and has to be kept in extreme conditions for it to break. It is 7 times harder than plexiglass, and is an optimal alternative to sapphire glass even if it certainly does not reach the same resistance level. Even though mineral glass is more expensive than acrylic glass, it is just as acceptable, and thanks to this it can be changed easily if broken.

Sapphire glass remains the best of the three as far as strength, resistance to knocks and scratches are concerned, but also because of its transparency. Even if the material is extremely resistant, however, it is not indestructible: with a well-placed blow the glass can shatter completely. As already seen, it is second in the Mohs scale, lower only than diamonds and higher than topaz. Corundum, of which sapphire is made, has an absolute hardness of 400 and is four times more fragile than diamonds, which have an absolute hardness of 1600. Compared with glass, though, it is three times stronger, which makes it the best material for watch crystals.

Sapphire glass production in detail: method and prices

Synthetic sapphire was invented by the French chemist Auguste Verneuil in 1902. The chemist invented an actual process for flame fusion that is still being used today, even if it has been modified slightly because of the arrival of new equipment.

The method invented by Auguste Verneuil involved melting materials in a furnace using an oxyhydrogen flame. In this precise case, aluminium oxide dust was melted to obtain sapphire. A long cylindric crystal was formed, which could only be cut with a saw having a diamond-tipped blade, because diamond was the only material (together with boron nitride) that could cut the corundum. After being polished, the crystal was then ready to be marketed.

Sapphire glass is not easy to make: it melts at 2040° centigrade (an extremely high temperature) but it remains stronger and more resistant than traditional glass.

When making a good watch, or even a good pocket watch, you can be happy with quality mineral glass. If you are looking for something better, though, sapphire glass has no rivals: even though expensive, it is a synonym of quality.

Another factor that can affect the price is the type of glass: flat or domed.

Flat crystals are the most common and the most practical, because they are less exposed to knocks. Their flat shape, however, makes them more fragile in the event of falls. An anti-reflection coating is also obligatory, to prevent the glass from reflecting the light and making it more difficult to read the dial.

Domed crystals are more resistant to knocks, but they project, making them easier to knock. The dome shape makes them immune to problems tied to reflection: there are many people who believe that an anti-reflection coating on domed glass is excessive. Domed sapphire crystals are more expensive than flat ones because of their shape and the related production difficulties: modelling a material that melts at 2040° centigrade is not simple at all.

In general, it is much more complicated to work sapphire glass than mineral glass, and this is why the price is much higher. It is also difficult to guarantee the absence of micro-cracks, which can weaken the glass. The technique and quality at the moment of processing are therefore extremely important for guaranteeing high quality sapphire glass, and they obviously have a strong effect on the price, which already starts off as being not for everyone’s pocket.

The history of sapphire glass in watchmaking

It is difficult to trace the history of sapphire glass in the watchmaking world. From what is known, it was used for the first time in 1932 by Jaeger-LeCoultre for some Reverso watches, one of the top models of the brand. Another 30 years passed before this type of glass was used on a large scale: it was the 1960s when using it for watches became more widespread. The first Rolex models with sapphire glass were the Oysterquartz, the Perpetual Datejust ref. 1630 and the Perpetual Date ref. 1530.

The most important companies decided to use sapphire glass for many reasons, but the strongest were better visibility and greater resistance to scratches. Being a very expensive material, its use was initially limited to elegant watches that were not suitable for everyone, but now includes almost any type of chronometer. An example is Rolex which, during the 90s, stopped using other types of glass to centre on sapphire glass.