[thejeff]

You can only use as many gems as your path level.

And only one more than the spell costs to boost your level.

So an A4 mage can use 4 gems, 3 for the cost and one to boost his level to 5 to cast it.

An A3 mage can only use 3 gems, which is enough for the cost but not to boost his level, and could only boost his level by one to A4 anyway.