[thejeff]

You control if a mage uses gems or not by giving them to him.

More usefully, a mage won't use gems if he thinks the opposition isn't worth it. This keeps him from using all his gems when attacked by a single scout, but will also underestimate thugs or SCs.

In general, only give mages enough gems to follow their scripts. After the scripts they'll waste gems doing things like summoning elementals that never reach the front lines.

I've never quite figured out when mages will use gems to reduce fatigue. It may be as simple as whenever they can, or it may have some lower fatigue limit.