Gem use question.

[Borg]

I can't figure out how to make a wizard cast a spell of higher lvl.

I have an Air1 mage, I have the air Gems, and when I pop open the roster to point out his battle spells to cast , some of the spells are shaded (like Lightning Bolt).

I guess he can cast a spell above his lvl if he uses a gem for it , Correct?

But HOW EXACTLY do I do that ?

Sorry for the newb question