That is an excellent question. Nothing seems to indicate they have metalcraft, yet they are also no stranger to "Thumper" items. And one has to wonder about the *origins* of Leafmen and whether or not they were full-sized humans at one point.
However, as everything else made by them appear to be made of naturally obtained and or growing materials, I'd think they are either a hardwood or even crystallized sap.
All of which is strictly conjecture on my part, but were I to write a fanfic set among them, that is how I would describe them.