not glass in my opinion, i work with every form of it and even with what come out out of the old furnace as part glass and refractory. have even made a few ancient glass furnaces and had all sorts of weird melted glass pieces. never have I seen a piece with any of these characteristics. but i have some really cool pieces I can show that you would swear were natural minerals or some sort lol, so I could very well be wrong. look at this weirdo from a furnace tear down.