If you watch your character's resistance list as you add or remove a crystal from the bag, you can determine what effect it has. The type of resistance bonus is given by the name (and colour) of the crystal. From small to largest, the magnitude of the resistance bonus is 2, 4, 6, 8 or 12.
Once you build up a selection of crystals, you can vary the crystals you use based on the damage your opponents in a particular area do, or simply use the strongest crystals for whatever that character's lowest resistances are.