About the glassworking aspect of it,,,, I don’t know myself. You should probably ask in the glass art forum, if you have not already.

…It is my understanding that the problem with glass-working is not heating it, but cooling it… If it cools too quickly it will develop cracks, so an annealing oven is used to cool it down slowly, from 1000F+ to room temperature, over a matter of days.

Also I can say that a lot of rocks and crystals will tend to fracture and may even explode (throw pieces) if heated with a torch. If you want a stone surface that can take the occasional blast from a welding torch, you must use firebrick or kiln brick. https://en.wikipedia.org/wiki/Fire_brick