Redstar45
The Anime/Special's canon know it all.
Well i didn't think that was in the game itself but then again the xy game is kind boring and the manga version is not that great as well .That's in the games according to Bulbapedia at least. Apparently Sycamore explains that Xerneas's and Yveltal's energy is infused into the stones becoming Mega Stones through AZ's Ultimate Weapon.
I will take animevese and my friend story idea over the Pokemon special xy and the games any days.