[Corinthian]

I am told that the AI will spend gems until the cost of the spell falls under 50 fat. Or possibly so that the caster remains under 50 fat. I have not tested this though.

But yes, an A1E3 rain of stone caster will easily spend 3 gems in a single casting of rain of stone, if allowed. This is why I used to use A1E2 casters instead if I wanted them to cast it more than once. A A1E2 caster can only burn 2 gems per turn and will thus have gems over for the next battle due to only burning max 4 gems before falling permanently unconscious.

An A1E3 caster would remain awake and continue to waste gems on misc spells until the enemy had left the battlefield.