[Arryn]

Adding further to Cainehill's reply:

Please note that the example on page 17 of the manual uses Fireball, a L2 Evocation spell that uses NO gems. Hence a L1 caster can use a single gem (his limit as a L1 caster) to boost his casting ability to L2. If Fireball needed any gems whatsoever, the L1 caster could not cast the L2 spell at all (unless via Communion).

A L2 caster could use 2 gems to boost his ability to L4, but only with zero-cost spells such as Fireball. If the L2 caster was casting a spell that normally needs 1 gem, then his limit would be only 1 extra gem (for L3 effect). And, as Cainehill points out, if a spell needed 2 gems, a L2 caster cannot apply any extra gems at all.