[Ygorl]

If I remember correctly (a forum search that I'm too lazy to do could corroborate), each spell gets 1d6 open-ended, with the high roll winning. To each roll is added the number of extra gems spent on the spell (above the normal cost - if you're using a site that decreases the cost, it starts counting from the *full* cost of the spell, not the reduced cost, so the site is useless if you want to add extra gems) and the caster's extra magic levels in the appropriate kind of magic.