AFAIK, the maximum number of extra gems is limited by the inital skill, ie. a 2lvl Air mage can only spend 2 additional gems. Furthermore I recall that only one gem adds to the skill, while all remaining only lower fatigue (as if raising skill).
I do not find the thread where this is posted, but it
should be (and probably is not) in the manual and it
probably is (or probably will be) written within liga's manual addenda.
PS: It's actually within the manual: see 6.4.7! I am positively surprised!