Never had 'of Air" ring which I wanted so I could get a measure of the Power of these type of rings.
So I decided to go to the Circus........
Got an "of Air" ring in the Desert after many tries.
Without "of Air" my Wizard was able to cast 12 hours of Fly time.
With "of Air" 18 hours.
Those rings are more powerful than I thought.

An "Of Water" item also makes your beacons last longer, at least in MM6. (I don't remember if they even had them in 7 or 8.)

The +5 bonus shows up when right-clicking a magic skill - see screenshot.

That's in MM7; bonus doesn't show like that in MM6.

I make the bonus in MM6 to be skill level + 50% (but halves round down). An "of Water Magic" ring let my character with master skill level 12 in water magic cast protection from cold as if level 18; but on another character, skill level 1 in water magic, it made no difference (duration stayed at 1 hour, didn't even increase to 90 mins).

Yep. Tried some other spells and "of xxxxx" seems to be 150% in MM VI. (Should have written "MMVI" in the first post).
That makes the rings more useful than I thought until the Blasters show up.

All "Of ____ Magic" rings, amulets, artifacts, and relics* multiply your skill by 1.5, rounding the remainder down.

If you are wearing an "Of Air Magic" ring and you have the Air Magic skill with 7 points, if you right-click that skill it'll show a +3 bonus, for example. In fact, you can right-click any of your skills to see if any of your gear provides a bonus to it.

* I am 99% sure that Rings, Amulets, and certain Artifacts and Relics are the only items that can have an "Of ____ Magic" modifier.

rmeakins has already shown that is so in MM7. And I've already replied that MM6 lacks that.