Season 4The Winx first found the ring exploring a farm from Roxy's childhood. The ring was so important that the Wizards of Black Circle aimed to get it first before capturing Roxy. The White Circle is the magical opposite of the Black Circle.
When Roxy finally transformed into a fairy, she fought the wizards greatly but it wasn't until she used the White Circle (she was scared at first), that the Wizards were defeated in that battle. It opened the portal to Tir Nan Og.
- The White Circle is probably a reference to Fairy Rings, which are naturally occuring rings or arcs of mushrooms. They were associated with lots of legends, and according to English, Scandinavian and Celtic legends they were made by elves or fairies dancing.