little green ring - glass, or stone?

This has been in my drawer for years now, having been found along with a handful of other various (costume, mostly) rings left behind in a desk drawer in a motel room I stayed at once. [ONLY once...long story there] I've never known what it really is or what its made of, but it is definitely not plastic, having the feel (and sound against a tabletop, gently) of glass, stone, or something otherwise fragile as all get out.

Does anybody recognize anything about it that might tell me? It is translucent when light shows thru it and shows a little bit of mottling, there are no marks. (if one could even mark it?)