Taking all your comments into account:
Earth
I would go for a mountain, definatly.. if by earth is meant the landscape. Its recognizable. Other things might easly be mistaken for the Nature discipline. However, if earth as in clay, we need something else...
Iron
I think I would go for the iron-ore. I'm not sure if I would draw it the same way as the much-wanted ingame iron ore (3 blocks of metal). I also thought of an anvil, but see note below about profession items.
Considering consistency I would use the 3 blocks symbolizing iron-ore also used in the games.
Healing
Now this is a tough one. I like the snake idea, however it is rather detailed compared to the other graphics so far, but ofcourse we could only do the top-part. An ankh, I didn't even know it was associated with healing

I'm not sure that is a clear symbol. I still like the simple heart symbol... though much used, but therefor also very clear, but nobody else seems to support that. I'm not sure if I would use any medical instruments or items... because in my understanding the healing discipline is about magic and spells (or not?) so I doubt is such profession items would be used. I was also thinking about a open-hand, with a radiance of light centered around its palm. I think that just a radiance like Elven suggested is too abstract considering the other icons. So, this one still stays undecided.
Dave: They would, as far as I know, be shown infront of the level of any of these skills in your skill-dialog.
I also was wondering, is there an explanation of all of these desciplines. what is their focus? Of some its quite clear, but others I'm not sure (like 'earth')...