[Lazy_Perfectionist]

To continue thejeff's example.
An A3 mage using 1 gem would get 33+enc fatigue
An A3 mage using 2 gems would get 25+enc fatigue
An A3 mage casting Summon Lesser Elemental cannot use 3 gems.

The reason for this is mentioned briefly in OmikronWarrior's post. This confused me when I first got the game, so I'll put a slight emphasis on him. I messed up in my first MP match several months ago, due to misunderstanding gem use.

"However, a mage can consume _only one_ additional gem beyond the spells requirements"