Gem Usage in Battle

[Chazar]

What are the criteria for gem usage during battle?

I just lost a fully equipped Rudra (and probably the MP game) against Vanheim 25PD. The Rudra was scripted to cast SoulVortex, Bless, PhoenixPyre, Bless, HoldTurn&Attack. Instead of using one of the 5 death gems in his pocket, he just went straight to around 50 fatigue by casting SoulVortex without a gem. Being highly fatigued, despite E4 bless and BootsOfTheMessenger, he did not use one of the 3 fire gems for PhoenixPower. Since the first self bless had actually hit him ok, instead of the second bless spell he choose to case some insignificant death attack spell without a gem, bringing him above 90 fatigue, which of course resulted in certain death.

I remember Dom2 mages being very eager to use up all their gems before casting spells that actually require gems, so I thought 5 death gems would encourage the Rudra enough to use one or two to lower the fatigue from SoulVortex, but apparently a 40 fatigue spell + encumbrance from HydraSkin is not enough to warrant gem usage in Dom3.

• Does battle gem usage depend on how fatigued the caster already is? (i.e. would the Rudra have used its death gems if I had first scripted PhoenixPyre, Bless and then SoulVortex)
• Is there a known threshold for using gems to avoid fatigue, besides 100+ fatigue spells?
• Does the number of gems in the mages pocket influence the likelihood of gem usage to reduce fatigue?

I would like to generally understand how the AI decides upon gem usage, the Rudra example is just an example from an ongoing game, which should otherwise not be of interest here.