Would you go for a new function for crystals, or an original function, not before seen in FF? It is possible that the collective belief of the crystals being made by the palace could be simply an inbuilt function. If you expand the extent of the Ivalice world, then why not extend the exsistence of crystals? They would possibly be required to hold the other nations. Or, make crystals the source of some power (corny, yes) that gives Ivalice an advantage. Maybe protection against its neighbours? The people knew crystals were vital to Ivalices protection, as shown when Cid mentions that the people will not object to stronger laws if they know the crystals have been assualted. Its up to you. But please dont choose some ultra corny, seen-it-a-dozen-times-before reason.