Feystone

Feystones are used by nobles to store mana and also to make their travel mount called the highbeast.

Highbeast
By dyeing the feystone with one's own mana, it can be morphed into the shape of an animal, that will move according to the user's will and fly through the air. The changes are according to the user's imagination. Nobles generally change it into a mount reminding of animals like pegasus', griffins or some other animal with wings. The Archduke's family in Ehrenfest generally create their beast modeling after a lion with wings. Aub Ehrenfest uses a lion with 3 heads.

Storage
Feystones can be used to store mana and can be used when needed in the future. If you exceed the feystone's capacity, it turns into gold dust. The feystones come in different qualities which means they can store different amounts of mana. The higher the quality the more expensive a feystone is.

Magic tools
Feystones are used in magic tools to enable their functions.

Acquire feystones
Feystones can be acquired by slaying feybeasts. Even commoners can acquire feystones be slaying weak feybeasts. There is a store on the west side in Ehrenfest's lower city which buys feystones.

Trivia

 * Rosemyne - when she was given a feystone and started to learn to expand the feystone, makes it into a balloon and as it grows bigger and bigger, she imagines it popping which it does . Ferdinand is quite annoyed as the stone is very valuable but to his surprise she changes what she has in hands, imagining clay, and goes around picking up the bits and puts them together which shocks him.