Chiming in with a love of that physics one.
While modern renditions of the four elements usually lump metal in with earth, back in the day, it was considered to be an even mix of all four (gold of course being completely and perfectly even). I'm tempted to say that if someone gets enough evocation refinements to be good at all four physical elements, they can take another refinement to have metal as a fifth - representing the channeling of all four - with a purview of perfecting and strengthening things (and allowing the use of the elusive healing magic).
I believe the djinn from the Quran were create from smoke and fire, so that would make thematic sense as an ancient 'element'. - Masurao
They were made from smoke
less fire* - although you could justify the same idea from that - normal familiar fire being a mix of elemental fire and smoke, with djinn being made from the elemental fire. Angels in the same scheme were made from light, which would be good for veils, illusions, and mind magic.
* Which I'm tempted (though not for a Dresden setting) to make electricity, with the City of Brass as a high tech realm, and the Baghdad Batteries a way of summoning djinn onto Earth.