I was in the market for a watch (what else) and needed some information on why some high-end watches didn't use sapphire watch crystals. I found the answer in an interesting message on alt.horology. That message cleared some of the fog on the issue of which crystal is better for some uses and why. I take no credit other than my cut and paste skills.

"I really can't tell you what Seiko is using for their crystals, but I think you can be pretty certain that if the Seiko model in question is moderately priced, the material used in the crystal is likely "mineral" glass and not synthetic sapphire.

To my knowledge, there are three materials commonly used in the manufacture of watch crystals. They are acrylic, mineral glass and synthetic sapphire.
Acrylic has been and is still used primarily for water proof watches. It is ideal for this application since the relative softness of the material makes an increasingly tighter seal as the outside pressure increases. Acrylic has also been widely used in low cost replacement crystals for vintage watches.
Acrylic crystals have the benefits of being water tight, durable and inexpensive. And although acrylic plastic is by far the softest of the 3 materials, it can be readily ground and polished if scratched.

Mineral is a term that is misleading in most cases. The word mineral would suggest something coming "from the earth" which is not the case anymore since synthetic minerals such as quartz and sapphire are far superior in purity than the natural forms. When the term "mineral" is used however, it is usually in the context of "mineral glass" which is nothing more than
glass. It is a rather special glass however, in that it is very clear and almost colorless as opposed to common "float" glass which is much too green,
when viewed from the edge, to be used in watch crystals. Mineral glass is among the materials I use in the manufacture glass pocket watch crystals. It
comes in sheet form, is brilliantly clear in appearance and can be formed and machined with beautiful results. The majority of mineral glass watch
crystals produced in the world today are made using this material, and while it is perfectly suited for this purpose, it has more or less the same hardness as common glass. To my knowledge, there is no surface treatment that will result in the surface being harder than the underlying glass, and surface tempering cannot be done to mineral glass in the range of 1.0 - 1.5 mm thick.

Synthetic sapphire is the hardest material used in watch crystal making. It is very hard and really very scratch resistant. I don't think you would find
a sapphire crystal on an inexpensive watch, but I could be wrong. The last time I looked though, this type of material was really expensive. The down
side is that it is also very brittle and shatter prone. Additionally, if it does become scratched, the only practical option is to replace it.

I have done a bit of research in this area, but I only know what I know, and I am certainly open to corrections from any experts out there."

Bill White
NAWCC # 0118120
Cazadero Railroad Crystal Co.
707-865-1938

choice for divers watches. Makes sense with the explanation. Comments I had read elsewhere led me to belive that sapphire was the crystal to be sought. It looks like the decision about which crystal to use is more tied to the application and cost more than anyhting else.

Imagine that - a practical rationale for crystal choice! Ya don't see that often in the watch world!

What you're more likely to see is: "These fine crystals are built at the molecular level by scanning tunneling electron microscopes using 128-node SP2-driven precision machinery. They are conditioned on a bed of platinum coated with diamond, then soaked in a gold-lined vat of Latour 61. They are then dried in a bushel-basket of saffron and then delivered in an Aston Martin Virage Volante Diamon Jubilee Edition to our team of handpicked engineers. Your precious crystal is then banged into your watch with a Sears Craftsman hammer because you're gonna scratch the damned thing anyway."

Hehehehehe....

-Zagi

nt

"Acrylic has also been widely used in low cost replacement crystals for vintage watches.
Acrylic crystals have the benefits of being water tight, durable and inexpensive. And although acrylic plastic is by far the softest of the 3 materials, it can be readily ground and polished if scratched."

I disagree, all high end vintage watches used arcylic at the time, as the trend was to use it then, Omega Seamasters and some Rolexes used it in the 60s and some models of Sinn and Kobold still do even though I don't prefer them myself.