Question about gems in combat...

[thejeff]

So, in a slightly different twist on the topic of gems in combat:
How many gems can a mage use above the cost of the spell? I'd thought it was only one, but I just had a Communioned up Dakini use 3 gems to cast Arrow Fend. It should have been A6 with the Communion, so only about 12 fatigue before gems and without the communion.
Do Communions screw up the algorithm?

My usual practice, if I wanted a mage to cast multiple gem-using spells, has been to give him one extra for each spell but the last. It's usually worked. Do I have to fall back to giving him Path gems for each spell to be sure?