[Vynd]

To elaborate further: When casting a battle spell a mage can use a number of gems equal to his level in that spell's primary path. The first gem will be used to raise his effective level in that path by one.

This is good in one of two ways. Either it enables a mage to cast a spell that he otherwise wouldn't be able to. Or, if he was already able to cast the spell, being at a higher level than the minimum required to cast it will reduce the fatigue that he suffers. That's what Agrajag was getting at.

But the mage can use more than one gem, if his skill in that path is high enough. Only the first gem will raise his level, but the other gems will reduce the fatigue that he gets from the spell. This is one of the ways in which you can cast spells with extremely high fatigue costs.

If there is a specific spell you want you mage to cast that is one level higher than what he could normally cast, give him some gems and script him to cast the spell. Note that if you want him to cast the spell repeatedly, he needs a gem for each casting.

Even if you do this, the mage may not cast the spell you want. There are a couple of reasons for this, as I understand it. One is that the AI that runs your side in a battle evaluates whether or not the enemy is really strong enough that gems should be used up fighting it. If your army is strong and the other army is weak it will not use gems. The other reason is that mages will not cast spells that will leave them too exhausted.