Unless it's a broomstick, but none of those have come out yet.
The thing that balances Mage out is the health. For each class that can use the gems to double damage, they gain the least health of anyone. (Mage's start out with low health, Their advanced classes have the lowest health gain, Necromancer's only gain 5 health, etc) I still agree that they're some of the most powerful players in the game, though, but I don't think nerfing them would do much good.