The glass like in the game is much like obsidian and forms from volcanic deposits deep underground...........end of science lesson 101.....LOL
You're referring to http://en.wikipedia.org/wiki/Volcanic_glass, of which there are several types. I always thought it was called glaes, but apparently that's not even a word.
Historically, some cultures referred to certain types of http://en.wikipedia.org/wiki/Amber as glass. This is especially true in the Baltic regions.
I suspect that based off the way it's used in most RPGs and because of the coloring of the glass weapons and armor in the last game, it's the volcanic type in this game. That would be the Eastern region of the map for Skyrim I suspect. Again, just a hunch, but that's where the volcanic activity is in Skyrim. It may or may not actually correlate in the game. It's going to be around Eastmarch if the game designers put the glass near the volcanic areas.