Beth She’arim was a cemetery located in Galilee. There is a cave of great antiquity.
In the cave was found a great slab of material resting under meters of silt and debris.
The link below is a direct page from the Corning Museum of Glass. This article tells the main academic story.
https://www.cmog.org/article/mystery-slab-beth-shearim
The update of March 2019 in this particular article expresses older and experimental transition.

Abstract:
The glass could be much older because of timelines established by academic procedures.
Certain materials are difficult to date, so surrounding artifacts, are used to give an estimate.
The main question is, “why so large”?
Nothing in the world except our large telescopic mirror glass matches its weight.
Quite a mystery.