Watch glass’ may also refer to a watch crystal, the transparent cover on a wristwatch.

Caesium fluoride sample on a watch glass
A watch glass is a circular concave piece of glass used in chemistry as a surface to evaporate a liquid, to hold solids while being weighed, for heating a small amount of substance and as a cover for a beaker. The latter use is generally applied to prevent dust or other particles entering the beaker; the watch glass does not completely seal the beaker, so gas exchanges still occur. When used as an evaporation surface, a watch glass allows closer observation of precipitates or crystallization, and can be placed on a surface of contrasting color to improve the visibility overall. Watch glasses are also sometimes used to cover a glass of whisky, to concentrate the aromas in the glass, and to prevent spills when the whisky is swirled. Watch glasses are named so because they are similar to the glass used for the front of old-fashioned pocket watches. In reference to this, large watch glasses are occasionally known as clock glasses.

Wristwatches usually come fitted with some form of glass, to protect your watch dial and allow you to read the time. A watch crystal is, for lack of a better description, the window of glass covering the face of your watch that keeps dust, water, dirt, or anything else from coming in direct contact with the dial and, by extension, the movement. Clearly, it serves an essential function.

Crystals usually come in one of three varieties—acrylic (or plastic), mineral, or sapphire. Every material has its good and bad sides that you might want to consider, as well as other factors, when buying a watch. Today we talked the mineral vs sapphire crystal most of the watches use these now.

Sapphire Crystal (What we use):
It is made synthetically and isn’t actually considered a glass but a crystal. The sapphire crystal is made in very high temperatures and it made up from crystalline aluminium oxides. These are then cut with diamond saws, grinded and polished to the finished “glass” product. Sapphire crystal can sustain scratches very well and it can only be scratched with materials, basically diamonds and especially hard rock types.

High-end expensive watches usually have Sapphire crystals and for a good reason. Sapphire is very strong and scratch resistant and is a more expensive crystal than the others. The advantages to a Sapphire crystal include its resistance to scratches, shattering, it can withstand cracks and a lower chance of breaking.

Mineral Crystal:

which is usually a common watch glass is scratched more easily, but it can take hits better. The same way acrylic glass, meaning a “glass” made of plastic, can take hits because of its flexibility, but is scratched very easily.

We see that being able to be scratched and being able to take impacts are opposite qualities. The harder and less likely to be scratched the glass is, the less it can take impacts. Hardness is measured with the Mohs scale of mineral hardness from 1 to 10.

Diamond is the most hard with its value of 10, it can scratch all other minerals. Mineral glass is around 5-6 and acrylic around 3. The Rohje Sapphire crystal is 8-9 on the Mohs scale, being nearly unscratchable.

Mineral crystals are actually a common glass crystal that has been treated with heat or chemicals to prevent scratches. Although it is not as resistant to scratches as sapphire, it is much better than plastic. The downside, if you are in extreme heat or cold and your watch hits against something it can crack or even shatter.

Sapphire vs Mineral:

If water gets on a sapphire crystal, it will come together forming a bead. If water gets on a mineral crystal, the water will separate in different directions.

A sapphire crystal can appear as a slightly tinted pink or milk white color, while mineral is often blue.

Sapphire crystal is cold or icy to the touch where the mineral crystal is not.

If you need more information about the crystal on your watch, there are machines that can measure the density, luminosity, hardness, and the index of refraction.

HOW TO TELL THE DIFFERENCE
As stated earlier, the primary means of distinguishing between different types of watch crystals is finding the price of the watch itself, which should be easy if you bought it from a trusted vendor. There is no other way to discern between different types of watch glass without scratching the surface, unless your watch has a sapphire crystal. If your watch’s crystal is acrylic, it will make a different noise when tapped on than sapphire or plastic will. You can also find the watch’s crystal type by Googling the case number, which can be found on the watch depending on the brand.