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"
Lazy Perfectionist, I think you are really wrong. I think the correct version is you can only spend one gem to raise your caster level. Additional gems still reduce fatigue.
KristofferO said:
You are on track. Max nbr of gems are mage lvl. This includes casting cost and increase lvl cost.
thejeffs example is correct.
Furthermore:
A mage can only use one gem to empower himself during a casting. This is the only way for him to reduce fatigue. He will not use more gems then one to reduce fatigue, and fatigue is only reduced as a side effect of him being temporarily more powerful.