Ironwood was sturdy, heavy, metallic lumber from ironwood pines which was completely stable in all temperatures. Sky pirates valued it highly for cargo as mentioned in Stormchaser. It was valuable for use in construction -- notably in the manufacture of skyship and skycraft flight-weights. In the Third Age of Flight, phraxmuskets were known to use bullets containing ironwood.
The reality of very different things yielding the same substance when processed and refined, such as iron ore and iron lumber is exemplified by sugar cane and sugar beet. Both result in granulated sugar.