There are 3 types of materials commonly used for watch crystals: plexiglas, mineral glass, and synthetic sapphire. The harder the material, the more scratch resistant it is. However, as the hardness increases it becomes more difficult to polish and more susceptible to cracking and shattering.

Sapphire crystals are the hardest and most expensive of the bunch, consisting of the same thing that the jewels in your watch's movement are made of: corundum. The only harder element is diamond. These crystals are 3 times harder than mineral glass and up to 20 times harder than plexiglas. Corundum is crystallized aluminum oxide (Al2O3) and is clear in it's pure state. Rubies and sapphires get their color from impurities - the red in ruby comes from traces of chromium and the blue in sapphire occurs because of the presence of iron and titanium. In 1902, a method to manufacture synthetic rubies & sapphires was developed. These man-made jewels were immediately used in place of natural jewels in movements. There were rare uses of this material for watch crystals as early as the 1930's but it wasn't until the 60's that synthetic sapphire crystals started to be used, becoming popular in the 80's. Sapphire crystals are impractical to polish because only diamond is harder. It's probably cheaper to replace the crystal than pay the labor it would require to buff out defects.

Mineral glass is very similar to the glass in a window. To the unaided eye it looks just like synthetic sapphire but is much less expensive. It's heat tempered for additional hardness, and Seiko goes a step further by covering it with a layer of synthetic sapphire. They call these hybrid crystals "Sapphlex". The CVD process can even be used to apply a diamond film to the surface of these crystals which increase the hardness to that of a sapphire crystal. Uncoated mineral glass can be polished using a paste of either diamond powder or cerium oxide.

Plexiglas, or acrylic, is the easiest to scratch of the crystal materials. It's also the easiest to polish - even toothpaste can be used! Basically, these crystals are all some form of high impact transparent plastic. They can be cracked but it takes a much more violent strike to break an acrylic crystal than a mineral glass or sapphire crystal. Omega used a version called Hesalite™ on their watches because it was so resilient and nearly unbreakable in extreme conditions.

Ω logo on a Speedmaster acrylic crystal:

photo by austin_p


To compare the cost of round crystals (called "lunette" - from the French word for moon), sapphire can run from $70 to well over $100. Mineral glass is typically $20 - $80, and plexiglass replacement crystals run about the same as glass. Special shaped or logoed crystals will naturally add to the price, but not necessarily a dramatic increase. Here's a Bulova shaped & raised crystal that only cost $30 in NOS condition in the original wrapper:




We'll use Mohs' scale to put each crystal material's hardness into perspective. The softer the element, the lower the number on the scale:

1. Talc
2. Gypsum
3. Calcite
4. Fluorite
5. Apatite
6. Orthoclase
7. Quartz
8. Topaz
9. Corundum
10. Diamond

As you can see, sapphire is made of corundum and rates a 9. Glass falls in the neighborhood of a 6 (+/- 1), and plexiglass ranks around 3 - 4.

Anti-reflective coating isn't used on acrylic crystals, but can be applied onto mineral glass or sapphire crystals. Sometimes it's done only on one side, sometimes on both the front and back of the crystal. AR coatings are made from oxides that have a refractive index somewhere between that of the crystal's index and air. This causes the light reflected off the dial to be less intense, which makes the dial more readable. Prescription lenses and sunglasses use the same coatings to reduce glare. One side benefit to putting an AR surface on a mineral glass crystal is improved scratch resistance.

 

Thanks, two very informative 101's before breakfast. I feel a little smarter starting the day off today.
Keep em coming!

 

Do the Omega Logos only appear on Hesalite crystals?

 

You could find them on virtually all Seamaster, Constellation, and Speedmaster acrylic crystals from the early 1950's on. I don't think you could put a logo on the inside of a glass crystal, and I'm not sure it could be done on a sapphire crystal either unless etched in.

 

Thanks - I was taking a look at some of my meagre collection and was starting to become concerned that I didn't see the logo through my 5x loupe.

Then it dawned on me, the watches that didn't have the logo were Sapphire crystal. Was going to post a question to understand if it was coincidence - but this timely article helped calm me down!

 

Are the glass crystals on thirties and older watches just that - ordinary glass?

 

Mineral glass = plain old glass....

 

This has been nice daily reading... We should get a sticky thread or something linking to the entire series

 

You mean like this one?

https://omegaforums.net/threads/horology-101-understanding-your-watch.76731/

 

These are so helpful, really great threads.

 

I'm going blind. Thanks

 

Is the green Rolex Milgauss just a color thing or is it functional?

 

There is a laser etched/engraved logo on the Limited Edition Railmaster’s sapphire crystal. It’s difficult to tell whether it’s on the inside or outside, I’m guessing outside as it’s still visible when viewed at very oblique angles.

 

Ooooo, I smell a bet. I'll take inside if you're saying outside.

 

Inside was my first thought as it would be easier to execute without worrying too much about process depth, but when I tilt the crystal over on my RM the logo stays visible so that’s why I’m going for laser etched on the outside rather than laser engraved on the inside.

Anyway, great 101 series!

A question: why don’t the hour and minute hands have that short counter balance element as used on most (but not all) seconds hands?

 

Because they don't spin as fast as the second hand does.