What's the best coal to use. I am in Australia and heard that Jarrah was great and left nothing behind ?

Well that depends on what part of Australia you're in. I'm guessing W.A. because that's where the jarrah is. Also assume that you mean charcoal and not coal.

I use found charcoal exclusively in my forge, and I have tries lots of different timbers. Seems to me all the hardwoods and gums work pretty well. Ironbark and dead finish I really like, but bloodwood (sparky) blue gum, lemon-scented gum, stringybark, all do well as charcoal. The only ones I have found that don't work are cypress (hot but gone too quickly) and she-oak (too soft and spongy).  I haven't used jarrah or karri because we don't get them over here, but I would assume they would be ideal. I reckon Gidgee makes the best charcoal  in Australia, but I haven't used it since leaving Mount Isa ten years ago. They reckon River Red Gum and mallee root work really well but they are out of my area.

Let us know how the Jarrah goes.