Gengar's Mega Stone
First, put up a Haunter on the GTS and ask for another Haunter. When you get your Haunter, it will evolve into Gengar. To get the mega stone, have Gengar in your Pokedex and go to the Hex Maniac to the left of the Team Flare guy in Laverre City. She will give you the Gengar Stone! Yay!
We have more cheats and tips for this game here, Pokemon Y cheats
Comments for this hint