Ring of fire, Ring of Air, Ring of ?

[Mithalwen]

I agree that the Three elven rings mirror the fate of the silmarils cast in to fire and water and borne up in the air. Earth seems a bit too static and somehow it seems to me that since the stones and metal came from earth, earth didn't need a ring... but that is just my impression. As for the Elessar, there are various versions of its origins but the ethos behind it is different. It was always intendend to be and instrument of healing power rather than commanding power so I don't quite see it fitting in with the rings.