[thejeff]

Gems used for the cost of the spell do not count towards boosting the level or reducing fatigue.
Each 100 fatigue is 1 gem.

So: the S7, 700 fatigue spell cannot be cast by a S6 caster.

Flaming Arrows can be cast by a F3 caster with 2 gems. 1 for the spell cost, 1 to boost him to F4. He could use a 3rd gem to reduce fatigue.