Good question. I have the clear and it looks like glass, but may well be some type of plastic, especially since they make them in different colors. I've been using it about 2 months and almost always with cinnamon. What will the cinnamon do to the tank? It's still clear looking.
If you want to figure out whether a tank is glass or plastic:
Take the tip of a steak knife, and try to scratch the tank. If it's plastic, it will make a groove (you might not be able to see it, but you will feel it with a fingernail). If it's glass, it will have no effect whatsoever.
HTH!