in the early entrance of santalune forest. Only on Y, post game.
Hope I helped!
The Heracronite is exclusive to Pokémon Y only. You can find the Heracronite in Santalune Forest in a patch of grass in front of the initial entrance to the forest (post-game.)
For more help finding the Heracronite click on this video here.
Note: It must be between 8pm and 9pm in the game for the stone to appear.
Hope I helped :)
if your game is y, you need to go to santalune forest, go straight forward from if you come from the entrance from aquacorde town and look to the left . you need the upgraded mega ring.
hope I helped:)