Restorer's Pledge

Also to note that while you're able to do Mount Hyjal without attunement after 2.4, you still need to be attuned to get this ring.

Much like the Violet Eye Signets, if you choose the wrong ring or the wrong quest, you cannot swap back at the next reputation level. However, once you reach "Exalted" the option to exchange your ring for the correct one becomes available.

You get Friendly reputation and access to the first ring as you complete Hyjal attunement with Soridormi ("The Vials of Eternity" quest).

Official site list of all the rings from reputation with Scale of the Sands (only obtainable from Caverns of Time: Mount Hyjal raid instance), including the last version at Exalted:

http://wow-europe.com/en/info/basics/factions/scaleofsands/index.html

Not aware of the last quest's name, but this ring's version when you reach Exalted reputation is this:

Band of the Eternal Restorer

Binds when picked up
Unique
Finger
+28 Stamina
+25 Intellect
Equip: Increases healing done by spells and effects by up to 64.
Equip: Restores 10 mana per 5 sec.
Equip: Your healing spells have a chance to increase your healing by 175 for 10 secs.

Which easily makes it one of the best (if not the best) healer rings available at the moment, item level estimated at 152. Would welcome solid info on the procc myself.

However, trash mobs in Hyjal aren't currently giving the reputation they should, which is supposed to be patched soon.