Sapphire glass watch crystal

How do you know if a particular timepiece, or one you own, has a plexiglass crystal?

• You might describe a plexiglass crystal as warmer to the touch
• Provides less vivid viewing, compared to the other two materials
• Has an abundance of scratches and clouding
• A better method to determine that it is indeed plexiglass is by tapping it gently with your fingernail, as it will produce a plastic-like tone
  

For the most part, you'll find plexiglass in vintage pieces. In cheaper watches though, it is also used for various 'tool' watches when an “unbreakable” quality is required.

Expect to find mineral glass crystals on a wristwatch geared towards those on a budget. That doesn't make its use an indicator of poor quality overall as many high-quality timepieces use hardened mineral crystals.

How do you tell if a watch has a mineral glass crystal?

• It will be cooler to the touch, like window glass.
• They should be clearer than plexiglass.
• They should have fewer scratches.
• The fingernail test works well, producing a higher-pitched sound.

How do you tell if a watch has a sapphire glass crystal?

• Cool to the touch
• Few, if any scratches
• Excellent clarity
• Try and scratch it with a metal object*
• You also can use professional machine to measure the density, luminousness, refractivity, hardness and some other technical information
• Color, the colour of sapphire is a little pink or milk white, and mineral glass often have blue color (as reference picture below)

*Caution! If the crystal is mineral glass or plexiglass, you might end up with a crystal repair bill.

AR Coatings:

Sapphire crystal is more reflective than mineral crystal due to its higher index of refraction. (1,8 compared to 1,47). Applying one or more layers of AR (anti-reflective) coating will limit this reflection to a very low level. Very often, lines and marks on the top surface of a sapphire crystal are mistaken for scratches but are actually the scratched AR coating, or just AR coating wearing off after time. An AR coating on the inside of the crystal provides a good AR performance while avoiding this problem. If you do scratch a sapphire crystal, then it cannot be polished away and will need to be replaced.

Waterproof gasket:

As both mineral and sapphire crystals are extremely hard, they have little or no “flex”. This means that when the crystal is inserted into a hard, steel or titanium watch case, it requires a gasket to ensure a waterproof seal.  These crystal gaskets are typically made of nylon, and are either flat, “I”-shaped or “L”-shaped. (see example above). The crystal is pressed into the case, using a crystal press tool. When it is pressed in, the nylon crystal gasket is “squeezed” between the crystal and the case rim, making a reliable waterproof seal. Over time, these crystal gaskets can become brittle and break, so it is a good idea to change the crystal gaskets every few years, to ensure the ongoing waterproof performance of your watch.