No glass. Just random open spaces. I dug about 3 blocks deep last night in my basement, so maybe 1000 cobble, and I put them in a furnace. I got to level 31 or something close to that. With the stone I fired, I put a few more levels on the top of my tower.
When I logged in today, all the stone I mined in my basement was returned. I went up in my tower to add some more stone, and I could see where there are 4 square holes where I had laid them last night. When I was doing the laying, I just ran around the circumfrence of my tower and added 2 blocks all the way around. I did this maybe 20 times, so maybe 40 layers I added. I don't get how the ones missing now are perfectly square and spaced like they are. If it took them in the order I placed them, it would just be levels lower and not whole sections missing.
Anyway, its very weird. I didnt loose any of the levels I gained from smelting the ores, it looks like most of the stone (if not all) I mined is still in the chests I was putting it in. So, nothing was lost, just have to fill in the holes. I will most likely just make them glass.