Depending on the hardness of the glass, it coule be quartz, topaz or corundum (incl. emerald)
(the greenish colour is a bit reminiscent of apatite, but that is not hard enough).
Quartz is most likely, and it appears to have been part of a vein. But the apparently amorphous structure is a bit of an issue (if it was a cryptocrystalline form of quartz such as chalcedony, I would expect it to be more opaque).
I think investigtion under a microscope (as a thin section, in polarized light) could be necessary to get a reliable ID.