[llamabeast]

thejeff has it right.

Gem usage:
- Minimum: Gem cost of spell
- Maximum: Gem cost of spell + 1
- Another maximum: Mage's level

Fatigue cannot rise above 200 from spellcasting. A mage cannot damage himself from excess fatigue. Hence very high fatigue costs only really reflect the number of gems required for casting.

Some consequences:
(Required facts:
Imaginary Spell A is A5 and needs 1 A gem
Fog Warriors is A5 and needs 3 A gems
Shimmering Fields is A6 and needs 2 A gems
Imaginary Spell B is A9 and needs 9 A gems)

An A3 mage cannot cast Fog Warriors, as his level cannot be boosted enough. He cannot even cast Imaginary Spell A - there is no way an A3 mage can cast an A5 spell.

An A4 mage can cast Fog Warriors and will need 4 gems (3 for spell + 1 to boost level)

An A4 mage cannot cast Shimmering Fields.

An A5 mage can cast Shimmering Fields with 3 air gems.

An A6 mage can cast Shimmering Fields using either 2 or 3 gems. Using 2 he will get 200 fatigue. Using 3 he will count as a level 7 and will only get 100 fatigue.

An A9 mage can cast Imaginary Spell B. Although the fatigue is listed as 900, he will only take 200 fatigue.

I am reasonably sure of all this. Hope that is helpful.