The Mage Stone

Magic skills — Illusion, Conjuration, Destruction, Restoration, Alteration and Enchanting — improve 20% faster.

Activating a standing stone grants its blessing permanently, but you can only have one stone's blessing active at a time — activating another simply replaces it, at no cost. The Warrior, Mage and Thief Guardian Stones near Riverwood are usually the first ones you reach.