Gem use, master enslave, high astral and oracle.

[Amhazair]

I'm not sure if some patch somewhere along the line changed the fatigue-reducing gem consumption mechanism? I seem to recall being certain that the stuff worked the way Soyweiser describes in his second post, and then at some point starting to be consistently surprised by casters using up more gems than they should according to my previous experiences. Long time ago though, so I'm absolutely not certain anymore.

[Peter Ebbesen]

Well, my memory says that the "you can use up to pathlevel gems to cast a spell of which one can be used to boost pathlevel and the rest to cover spellcost and fatigue reduction" mechanic has been in the game since Dominions 2, but it is such a long time since I played Dom2 that I may remember wrong. I'm pretty sure it has been that way since launch of Dom3, at the very least.