Victorian Adjust Ring with LARGE Green Stone

The ring, is pretty captivating. Some of the "gold" has chipped off in places, but its a pretty well made ring. The band adjusts and stays put, it does not pull from the setting. The stone, is 20mm across. Its green and foil backed. I looked at with a loop, and Its flawless. I am guessing glass or possible rock cyrstal? I would love to anything about it!!!