@High Seas,
I agree that it is more a semantic issue. You could say that the steel in an I-beam with a boulder sitting on it is unstable if the stresses on it are high enough that just a little more force would cause it to fail. I think that is how people might think of "unstable": a situation where a little more energy causes a system to fail and release a lot more. A boulder balanced on a hill top is unstable. 4+% Hydrogen in air is unstable. Back to the I-beam example, I would say that the material is not inherently unstable but that it is stressed close to its failing point. Glass, even impure glass used in art work is not inherently unstable. Window glass and drinking glasses are about as impure as you can get. We don't really consider those things to be unstable although most people know from experience that you shouldn't rapidly heat or cool them. I challenged the original statement because I don't want the idle reader thinking "oh, my windows could crack at any moment."
