[Truper]

You can only boost one level, and use additional gems to reduce fatigue. Also, the total number of gems used may not exceed the mage's skill in the path. This becomes important for spells which take gems to cast. Consider the spell Summon Lesser Fire Elemental. It has a path requirement of 2 Fire, and a gem cost of 1. A level 1 Fire mage cannot cast the spell even if he has two gems, since the spell itself requires one, and a level one mage may only spend one gem at a time, meaning he cannot spend an extra gem to boost his skill. A level 2 fire mage can cast the spell for one gem, incurring the full fatigue of 100. If you give that mage 2 gems, he will use both, effectively raising his skill to 3, and he will therefore incur half the fatigue, or 50, and avoid becomming unconsious.