To get crystals, you must goto a guard at the border of the area that your kingdom reigns over, and ask the guard ro cast a spell called 'Signet' on you. 'Signet'' is only a temporary spell, and will only last for a given amount of time. Once the guard has casted Signet on you, you must then battle enemies in the area to get as many crystals as you can before the spell runs out. There are 8 crystals in total in Final Fantasy XI. Each of them have different elemental properties. Below, is a list of the 8 crystals available in Vana'Diel, and their effects.

Crystal	Usage
Dark	Has the ability to damage an object severely to an unusable state.
Earth	Used for combining items into one.
Fire	Can burn or heat objects.
Ice	Has the ability to make things hard or freeze them.
Light	Used to enrich an object.
Thunder	Used to finely cut an object to use with the Earth Crystal.
Water	Has the ability to turn things into liquid.
Wind	Has the ability to cut things.