watch-crystal

Question: Are crystals … crystal?

2021/10/27

The watch “crystal” is the hard, transparent material covering and protecting the watch dial and inner workings from the elements and a variety of life’s encounters. And to answer the question … NO … they are not made from crystal.

I have learned that there are actually a variety of materials watch crystals made out of. Each has it’s strengths and weaknesses.

Crystal Materials

Plastic/Acrylic

Acrylic watch crystals were popular in the mid 1900’s and became an important component in most watches of the day. It was sent to the moon on the Omega Speedmaster since it could not shatter and and posed no risk of glass shards floating around the spacecraft with the astronauts(bt).

  • AKA – Hesalite, Plexiglass
  • Pros
    • Inexpensive
    • Flexible; doesn’t shatter
    • Replacement crystals can be custom cut and shaped(es).
    • Scratches can be buffed out.
  • Cons
    • Not scratch resistant.
    • Can become brittle over time.
    • Higher level of maintenance.
    • In order to maintain water rating needed to be routinely replaced.

Mineral Glass/Hardened Glass

Mineral glass is a term that confused me as it suggested something in particular; a specific formulation of glass and some other material but this is not the case. It is simply a term that is applied across the spectrum of standard glass used in watch crystals. In the mid-1900’s, when plastic was a popular material for watch crystals, the acrylic crystals became referred to as “watch glass”. The term “mineral glass” likely emerged out of a need to differentiate actual glass crystals from the synthetic plastic crystals.

It is possible for glass to to be chemically treated and be further”hardened” using proprietary processes. Despite the different techniques for hardening glass, the results are categorically the same. More durable, scratch resistant crystals.

  • AKA – Wittnauer’s Mineron™, Seiko’s Hardlex™(zl)
  • Pros
    • Scratch resistant. Hardened/Mineral glass ranks around 5 on the Mohs Scale; the same as tooth enamel!
  • Cons
    • Although it’s tougher to break, when it does, it’s more prone to shatter into splinters. Glass splinters floating within an automatic mechanism could cause more extensive and costly damage.

Synthetic Sapphire

As far as I have been able to uncover, Synthetic Sapphire is widely and often deceptively used term.

On the one hand … I have heard it used to legitimately describe Seiko and Invicta’s technology of ‘fusing’ sapphire with hardened mineral glass, creating a material more durable than simple glass but less so than true sapphire.

On the other, less reputable dealers have been reported as associating the term with hardened or regular mineral glass.

  • AKA – Seiko’s Sapphlex™(zl), Invicta’s Fire Fusion™
  • Pros
    • Most resistant to scratching
    • Clear for easy readability
    • Requires little maintenance.
  • Cons
    • Expensive
    • Shatters when broken

Sapphire

Sapphire is the hardest and most scratch resistant of the crystal materials and the most expensive. Sapphire’s used in watch crystals are typically man-made

  • Scratch resistant. With diamonds ranking 10 on the Mohs scale, sapphires weigh as very hard; registering 9 on Mohs Scale (zl). In fact this material is so durable it’s also often used as “jewels” in the watch movement.

 If the crystal shatters

If the crystal should shatter then it’s important that you pull out the crown(bt) and stop the mechanism ASAP to prevent further, more expensive damage. Contact  local watchmaker and have the crystal professionally removed and replaced.

If it’s a cheap Chinese watch like the pieces we review here, and the crystal shatters then you just throw it away. For the cost of replacing the crystal on a quartz watch, you could purchase probably 5 or 6 new watches. If you’re replacing the crystal on a mechanical watch where the watch would need to be cleared of any potential splinters and fragments the repair cost would probably be enough to purchase many, many more cheap Chinese watches. Rolex charges about $800 just for the service – then about another $150 for the [Sapphire] replacement(ws). So … that’s a lot of cheap Chinese watches …

 

Crystal Shapes

I originally thought that watch crystals were all intended to be domed, but have learned that that is not the case. GS Supplies, a popular manufacturer of watch crystals manufactures a variety of different shapes. I’ve listed a few of them below(gs) …

  • Flat top
  • Slight dome
  • Low dome
  • High dome.
  • Extra high

Identify Your Crystal

Particularly in the cheap Chinese watch industry there isn’t a lot of discussion about the material of the watch crystal. Or perhaps you’ve received a watch as a gift or second hand and the type of crystal was not revealed. How do you find out what the crystal material is?

I have found limited information on this topic but will share what I have found. There are a few tests that can be applied to assist in determining what your watch’s crystal is made of.

Water Test

The water test distinguishes glass from crystal by observing the way a drop of water stands on the crystal.

  1. Sapphire – The water will “bead” up on the surface. The sides of the “bead” will actually expand out over the foot print of the droplet. Literally the way a bead would look.
  2. Glass – The water drop will “puddle”. This is where the footprint of the water droplet will be the widest part of the droplet, spreading out wider that a bead would.

Sound Test

Each material makes a decidedly different tone when tapped gently with an item such as a fingernail or so.

Sapphire – Makes a lower, duller sound when tapped with a fingernail.

Glass – Makes a higher pitched, “clinky” sound when tapped with a fingernail.

While these tests are a good way to eliminate the presence of sapphire , I am not sure how or whether these tests can distinguish …

  • Mineral/Hardened glass from regular  glass
  • Synthetic Sapphire from solid Sapphire
  • Acrylic/plastic.

I’ll continue to look for more tests.

Video Links

These videos are not embedded and will open in a new tab/window.

Sapphire Glass VS Mineral Glass (Water Test)

References