Gem Usage Quirks?

[mivayan]

Ironhawk said:
5F, casts Firestorm (5F, 500 fatigue, 5 gem cost), and gets only 200 fatigue! How is this possible?

It happens often, a mage casting a spell which should give him/her/it 300 fatigue (or 500) - and still having 200 fatigue afterwards with no hp lost. But sometimes they dont.

I thought it only worked when you have 0 fatigue before, but you said on the irc channel that he had cast phoenix power first.

Basicly... exact mechanics are unknown.