Best summoner Ring in game. Makes free Carbuncle possible.
Also, know it's impossible to reobtain. There's no way to repeat "Mama mia", there's no way to trick the game to grant you a new ring. Only hope is to GM call at most a week after you dropped it. Even then, it's not guarenteed you'll get it.
"Free" in the sense that you don't lose MP with Carby simply needs:
Carbuncle Mitts,
Light Staff,
A SINGLE piece of refresh/avatar perp - gear (looking at you, Austere Robe),
Auto-Refresh trait.
You can get (and keep) a free Carby from as early as lv51. All these others piece are definitely nice to have for other avatars, but they're definitely not needed as people seem to imply if you have the Mitts.