The Spirit Crystal Card was not a naturally occurring card, but a card made from a crystal and inscribed with a certain inscription designed by the bank itself.
This inscription could not be copied by others, and only the bank was able to create these special Spirit Crystal Cards.
The cards were handy in such a way that when they absorbed the energy of the various spirit coins, the coins themselves would also be absorbed by the card, but instead of being placed alongside the energy, the coins themselves had been sucked into a specific space made solely for these empty coins.
The card owner would then be able to hand in these empty coins when visiting the bank branches, and the bank would pay a certain fee for the absorbed coins to ensure that the cardholders had a certain incentive to send them back.