Gem use, master enslave, high astral and oracle.

[Squirrelloid]

Originally Posted by thejeff

Just to be precise, empowerment, magic items, or booster spells all allow you to use more gems.

I'm not sure about communions. They should, but I don't think I've ever confirmed it.
It would be nice if the AI was smart enough not to burn extra gems to reduce fatigue, when all the fatigue is going to the slaves, whom the extra gems don't help anyway. But it's not.

Communion spells let you use more gems if they raise your actual caster level (ie, masters) - not just your effective level for fatigue soaking (ie, slaves).

This is pretty obvious if you consider the astral communion whose raison d'etre is to cast master enslave with a less than S8 (base) caster.