The Firium Z (Japanese: ホノオZFlame Z) is a type of Z-Crystal introduced in Pokémon Sun and Moon. It is the type-specific Z-Crystal associated with the Fire type and allows Pokémon to upgrade their Fire-type moves to Z-Moves.
Unaffected by item negating effects (such as Embargo and Klutz). Item manipulating effects (such as Trick and Fling) cannot remove a Z-Crystal from a Pokémon.
In SM125, a young Professor Kukui was revealed to own one, which he lent to Ash, for him and his Torracat to perform Inferno Overdrive with. Later in the episode, Ash was given his own Firium Z from a TotemTrevenant after Ash helped to cure Trevenant of its cold.