They seem to be tied to the Blessing of Light. When we have the Blessing striped we see the crystals in our vision thing become colorless. Either that means they lost there power, which I don't think is the case, or the Blessing of Light is what enables us to tap into their power. Don't the Ascians at some point refer to them as the Crystals of Light? Ramah seems to think them extremely significant considering his reaction to seeing we were given his one by the Sylph and its also curious that Garuda was so badly effected when we basically forcibly drew hers from her.