The Ring of Resurgence can be used to restore all of your spells. Once used, it has a 6 hour cooldown before it can be used again. It can be obtained directly with Diamonds in the Magic Shop or as part of the Conjurer Pack in the Magic Shop. Once obtained, your Heroine will always have this item.
The beautifully set Mana Dragon gallstone was delicately extracted from a live host. The ring can restore all of your spells every six hours!
If you have the diamonds to spare, this is a great item. But it should probably be the 4th item you spend your diamonds on, behind Arai Cloak, Vorpal Buckle and Ring of Damage. The seemingly best use of this item is immediately after you use Intensify and Invigorate, allowing you to cast both again, thus allowing you to cast 4 intensifies at once.