PokeIdle
PLAY FREE Charizardite in PokeIdle: Mega X, Mega Y, and the Missing Drop
The question is usually where it drops. That cell is blank on purpose. The forms and the version split are the part I can source.
Choose the form, then the game
On Pokeidle.io both stones are real names in the megas module. X turns Charizard into a physical Fire/Dragon attacker. Y keeps Fire/Flying and pushes special attack to 159. I would pick Y for a special move set and X for a physical one, and I would not pretend a drop guide exists until the recipe is public.
Pokeidle.net reaches the same two stat lines at level 100 and then keeps them. You never buy Charizardite there. Poke Idle World's item list does not contain the stone, so a World video that tells you to farm it is talking about a different game. The species context is on Charizard.
Compare the tank mega on Venusaurite. The rule for every mega item on this site is the same: form and stats if the module lists them, blank drop if it does not. Evolution rules explain why net never asks.
Keep reading
FAQ
What does Mega Charizard X do?
Fire/Dragon, stats 78/130/111/130/85/100. The attack stat is the reason to pick X.
What does Mega Charizard Y do?
Fire/Flying, stats 78/104/78/159/115/100. Special attack is the reason to pick Y.
Where does it drop?
Not published. Io names the stones. It does not publish the fragment recipe in the files I could read.