What's interesting is I got a lot of the same possibilities that person who made that list came up with. The most notable being obsidian, pumice, and crystal. I just decided to replace those three with something more mundane because... well, obsidian and pumice seem a little too specific, considering the nature of the Inner Planes. Those two would seem to be more like by-products of pockets that formed on magma or the like. Crystal reminded me too much of mineral, and ice itself was more like cold crystal than anything, so I took that one out of my brainstorm as well and replaced it with snow, which is much simpler anyway.
Even though I'm keeping my list somewhat the way it is (with a few switcheroos whenever I think it necessary), I like this site. Just bookmarked it.