[Twan]

Yes I noticed the same thing, mages often use more than one additional gem, in addition to base gem cost of spells requiring gems (and this even when they have the level for the spell).

If only one can be used to reduce fatigue and boost level at the same time, there should be a bug somewhere.

I wouldn't be surprised to learn that they use another time the base gem cost of the spell to reduce fatigue, if they level and number of gems is sufficient, instead of base gem cost + just 1 gem to empower/reduce fatigue.