gems etc in combat

[Aezeal]

In my SP games I usually don't let my commanders carry gems etc because they tend to blow them even in small battles.

Of course I see the benefit of using battle spells that cost gems since they usually help your army a great deal. Especially in large battles the whole battlefield spells etc can be usefull.
In my MP games I can certainly see that using mist, storm etc in the LARGE battles can turn the tide of battle and save the lives of many of my skinshifters.

BUT if I give my commander 10 airgems (midgard is the nation of the moment) HOW do I see to it that he ONLY uses them in the large battles with 100 + troops and not in the small skirmishes.

Obviously start of game I just don't give them gems.. but how do I do that later.. when I want to bash a few small armies and then for example besiege a larger force somewhere. How can I see to it that when I load the gems on the mages they still have them when in counts?

Any advice would be appreciated.