Why not just have NPCs avoid mustard gas until it dissipates?
It seems kind of silly for them to stand around in it just because they don't know where it came from. If they end up in a cloud and don't have anyone to be pissed off at they should just move to a nearby square until it's safe to move back to their original spot.
But regarding stones, I'd love to have Malachite in IVAN, since I think IVAN doesn't have any green material yet (besides vomit).
Though I'm still all for making Valpurium green like in LIVAN. It seems far more fitting for some magical Frog God metal than the current golden color. Gold is too cliche anyways.