Contrary to what one might infer from the name, sapphire glass is not glass but consists of synthetically produced sapphire with a crystalline structure. The sapphire is a rare gemstone, which in its natural form today comes from Sri Lanka, India, Australia or the USA, for example.
Sapphire glass is particularly used in high-quality watches because it has what is known as a Mohs hardness of 9 (10 being the highest) and, simply put, only diamonds are harder. It is therefore not only valuable due to its complex production and structure, but also exceptionally hard. Although sapphire glass is not scratch-resistant, it is extremely rare that scratches are caused by particularly hard materials in combination with strong use of force. Scratches are often mistaken for abrasions on aluminum, which can be quickly and easily removed with a rubber band. The sapphire crystal can break under very strong local pressure with a hard object.
Why doesn't every watch manufacturer use sapphire glass for their models? On the one hand, it is complex and expensive to produce and on the other hand, the crystalline structure causes a stronger refraction of light and thus also a stronger reflection. Only new types of anti-reflection coatings can eliminate this disadvantage, but they also entail additional costs.
In comparison: mineral glass
Mineral glass consists of silicon oxide and can be refined or further hardened by adding various oxides. However, even after finishing, the mineral glass does not achieve comparable scratch resistance to the sapphire glass. As an advantage over plastic glass, it is significantly harder and does not yellow over time. However, mineral glass can break into the smallest glass splinters, which in the worst case can damage the movement.
In comparison: plastic glass
Plastic glass was first used in watches and is also known by the terms hesalith glass, plexiglass or acrylic glass. The original version of the first watch on the moon, the Omega Speedmaster Moonwatch, had and still has what is known as Hesalith glass, since the plastic glass cannot splinter into the smallest parts and therefore poses no danger to sensitive instruments. Further advantages are the low weight and the extremely low light reflection of the plastic. Compared to sapphire glass and mineral glass, however, it is much more susceptible to scratches and can let through small amounts of water under high water pressure. Fine scratches can be polished out again on plastic glass because it is relatively soft. The glass can yellow over time due to exposure to UV rays.
Currently, only a few manufacturers worldwide are able to produce sapphire crystal in complex shapes, this is especially true for domed sapphire crystal, which is still often said to be unworkable today. The producers are based in Switzerland, France and some in Asia.
Another important innovation of the 21st century is the scratch-resistant coating against light reflections. With a double-sided coating, the light is hardly reflected and a watch glass is hardly recognizable, which achieves a particularly striking effect with a domed sapphire glass.
In the meantime, the manufacturers of well-known luxury watches almost exclusively use sapphire glass for their watches, as it has clear advantages despite the higher costs and increases the value of the watch to a certain extent. The models shown have sapphire glass, which contributes to the high-quality effect of the watch.
The valuable gemstones with a blue color are imported from Australia, Sri Lanka, India, Africa or the USA. The world's largest sapphire is estimated at around 90 million euros and was found in Sri Lanka.
However, sapphire glass is not the mined sapphire, but elaborately produced crystalline glass that resembles the structure of sapphires. Briefly explained, a small piece of sapphire is heated with aluminum oxide over 2,000 °C until a glowing glass bulb is formed. The resulting sapphire glass is cut into slices and then ground and polished. However, this can only be done with diamond discs and diamond powder, otherwise no material would be hard enough. The exact manufacturing method and mechanics are known or possible only to a few manufacturers on the market.
At first glance, it is difficult to tell whether a watch glass is made of valuable sapphire glass or cheaper mineral glass. Although there is a very high-quality effect, especially with curved and double anti-reflective sapphire glass, which clearly emphasizes the dial, this effect could also be produced from a distance with plastic glass.
Thanks to the unique crystalline structure of sapphire crystal, a simple home test can quickly confirm authenticity:
As a direct comparison, it is best to take a watch with mineral glass or a conventional glass. First, the surfaces of the watch with the sapphire glass to be tested and those with mineral glass should be cleaned until there are no more grease residues and dirt particles. After that, both clocks can be placed next to each other.
A drop of water is now placed on the surface of each of the two watch glasses. This can, for example, be done gently with a fingertip previously moistened under the tap if no pipette is available.
If you now tilt both watches at the same angle, the water droplets run off the conventional glass or mineral glass much faster and also leave a streak. Due to the crystalline structure of sapphires, the water drop on the sapphire glass runs off visibly more laboriously (only from a steeper angle) and leaves no streaks.