[Taqwus]

Any caster who has gems in his inventory -- gems of the primary path of the spells he casts, anyway -- will automatically consider using them.

Using gems above and beyond the base required raises the effective level of the caster for fatigue reduction and effectiveness, IIRC. Having one level above what's needed cuts base casting fatigue (NOT including encumberance fatigue) to 1/2 normal, two levels 1/3 normal, etc.