An additional discussion on watch crystals
In addition to the question of materials that a crystal is made from (or made of?), there is the issue of installation into the watch case, which is also material-specific.
Plastic crystals are resilient / flexible, and they can be pressed into a case using special tools that compress or flex the crystal so it will go in, then it "springs" back out to hold itself in place.
One type of tool squeezes from the outside (see below)
Other tools press on the center of the crystal which causes the outer rim to flex inwards.
Both hand and bench-based presses are available.
Naturally you must avoid damaging the case which doing these operations. So a case holder, tape, careful handling, etc are required.
Glass crystals, on the other hand, unlike plastic are NOT flexible, can easily BREAK if you press too hard or with the wrong tool (bin there, dun that!), and often must be sealed with a gasket.
It follows that you must have exactly the right dimension glass crystal -- at least the diameter must be exact to within .1 mm -- the thickness may vary in some cases without an issue arising.
When crystals are arched, it's not just for visual effect, such as a box crystal, but also to clear the hands beneath it.
Watchmakers must have an annoyingly large inventory of crystals to keep their workflow going. I had a tiny aftermarket repair business and this is the inventory I had on hand. We still had to order about 1 out of 3 crystals.
We even had to replace a few rear view ports -- but they are much less frequently damaged and the technical demands on this glass are far less than those concerning the front crystal.
Here are a few variables concerning crystals.
Silicone gasket broken! Crystal good but now can't finish the repair and put it back in without this little string!
Glass goes into bezel, not the case.
Crystal shapes on Grand Seikos
A Watchmaker nightmare - thick glass, beveled edges, sharp corners, glass curved in 2 dimensions.
Here's a nice and easy replacement.