Alain owns a Charizardite X that he obtained from Lysandre along with his Mega Ring. Alain's Charizard wears it around its neck. After realizing Lysandre's true intentions and the error of his own ways, Alain disposed of the Charizardite X sometime before Till We Compete Again!, promising to start from a clean slate.
In Pokémon Origins
In File 2: Cubone, Mr. Fuji gave Red a Mega Stone and a Key Stone, although he did not tell Red what they truly were, merely saying that he would find out soon enough. In File 4: Charizard, during Red's battle against Mewtwo, one of the stones was revealed to be a Charizardite X from Kalos. Together, it and the Key Stone triggered Red's Charizard's Mega Evolution, allowing it to Mega Evolve into Mega Charizard X and weaken Mewtwo to a point whee it could be caught.
In the manga
A Charizardite X in Pokémon Adventures
In the Pokémon - The Legend of the Dragon King manga
In Malamar Traps, X's Salamè was revealed to keep a Charizardite X hidden within his tail flame, explaining his habit of holding onto his tail. After evolving into Charizard, he became able to use the stone to Mega Evolve into Mega Charizard X.