An Elf's Spark igniting is seen as a second coming of age, and the lasting effects, either visually, physically, and/or magically, vary heavily. Which brings us back to the Hanafuda Earrings, a prime example of items being influenced by the event. As a result of ignition, the design on the cards will change completely to reflect the newfound connection between the Elf and their core element. This change gives off a surge of elemental magic similar to an upcasted Thunderwave in what we can only assume is an effort to protect the user from whatever threatened them. From this point on the earrings become magical. The buffs, bonuses, and boons granted by the item vary, but are always related to the element. The most consistent effect is a spell or ability unique to the user, much like Runic Gear, which is communicated through the card's new design.
Demon Slayer is awesome, and it's nice to read something connected to it, even if like you said, they don't really do anything in the show. I really like what you did with them here, tho!
Thanks so much!