The gem is only needed if one is using Magic Duel Mod.
Oh, and AI does NOT understand that it might not be the best idea to not duel with someone who has high astral magic. I found this out when my F3S2 Mystic (first I recruited in that game! woohoo!
) magic duelled a blind, 20hp left S6 Cyclops of the enemy... and died (
) !