Looks like a lump of glass from a glass factory. My Grandfather had a ton of it. I am not saying it is not a real stone just wanted you to know it 'Looks like" glass from the pics. Hope it is a real stone for you !
Definitely Slag glass from a factory. (the bubbles give it away.) Slag glass can still have some value, there are people who buy pieces like that if you can find them.....
I used to know a youngster who was a very accomplished knapper. Mostly obsidian, but he also worked slag and waste glass from a glass school, plus several glass artists who would melt waste into 10 - 20 pound blobs for him. Lost touch a few years ago when the stroke visited me.
Can be from iron furnaces, the silicates would accumulate on the top of molten ore, and would be scraped off and tossed, depends on the impurities in the iron ore, what the color of slag would be, sometimes it would be almost a beautiful cobalt blue, but mostly it's various shades of green, often iron slag would be used as fill on railway beds,you can walk just about any railroad and find this,