[tinkthank]

This might bring up some balance issues, but is just a thought, inspired by the unit description "spends her time gathering herbs..."

Why not? Why not an alternative method of "gathering" gems?
Nature mages would receive the action "gather herbs"; much like Blood Hunters when Hunting, they would scavenge for nature gems, only they wouldnt cause any uprising; their chances for success would be higher in forest provinces and lower in waste ones; higher nature skill = better chance of success and # of gems / attempt (like BH).
A similar routine could be done with Necromancers and corpses to harvest death gems -- I believe there is a spell whose name I forget involving Ravens which is ficitonalized to do just this.
A mage with both Nature and Death skills would not be able to do both in one turn; each harvesting option is a discrete action.
I suppose similar fictionalizations could be found to allow for searches for other types of essences.
Well just an idea.