If held by a Charizard, allows it to Mega Evolve into Mega Charizard Y if its Trainer has a Key Stone. Each Trainer can only Mega Evolve one Pokémon per battle.
Unaffected by item negating effects (such as Embargo and Klutz). Item manipulating effects (such as Trick and Fling) cannot give Charizard a Charizardite Y or remove a Charizardite Y from Charizard.
Outside of battle
If held by a Charizard, allows it to Mega Evolve into Mega Charizard Y when it performs a Spectacular Talent during a Pokémon Contest Spectacular while its Trainer has a Key Stone. Charizard will remain a Mega Charizard Y until the Talent Round ends.
In A League of His Own!, Trevor was revealed to have obtained a Charizardite Y before the events of that episode. His Charizard wears on a ring around its tail.
In the manga
A Charizardite Y in Pokémon Adventures
In the Pokémon - The Legend of the Dragon King manga