There are three basic types of crystals.

Plastic/acriylic/plexi – different names for the same basic thing, plastic. It scratches easy but it can buff out easy. Very shatter resistant and in my experience all of them are domed at least a little bit. You really only replace plastic with plastic.

Mineral/glass – It is simply a round piece of glass. It doesn’t scratch as easy but you for all intents and purposes can’t buff it out. You just replace it with a new one for a few bucks. It’s harder/more brittle than plastic so it can be shattered by sharp knocks.

Sapphire – Same as mineral but even more hard/brittle. So it is highly scratch resitant, but can shatter the easiest(I have never seen it, although I'm not an expert).

Mineral and sapphire can be swapped for each other as long as they are the same size. A generic mineral crystal is less than $5 US while a generic sapphire is at least $40 US

Eddie

: A few weeks ago, I posted this...
: http://www.chronocentric.com/forums/heuer/index.cgi?md=read;id=51482

: I'm wondering if the experts know what's the shape of the crystal.
: I see it domed but it was previously buffed... I'm wondering if
: this used the same crystal that an 1163 Siffert has where it
: rises and bends way above the Tachy...

: Trying to see if I may be able to source from Nakai a crystal but
: not sure if an 11630 crystal work either...

: Let me here the experts...