Quick question how do you even get the mega stones ? I have been searching for the code but the only thing I found is for super mega emerald, and even with that the stone didn't show up
They are around the world, it's one of those things the dev should fix and it's adding in game some guide or help to where to find new things because it can be too random and this is big game where we also tend to skip things since we already played it.
Some mega stones i remember:
Starters mega: Fallabor town, go to the left and down a little, there should be double combat and a old man next to them, said old man sell the stones.
Gardevoirite stone: For Gardevoir, you have to fight in sootopolis a combat in one of the houses (north zone), so you need to solve the legendary fight problem there first.
Lucario: Meteor falls, you need waterfall skill to go up tho.
Mawile: Its in that weather building where you defeat the redhead from aqua, its in the end part where the scientists are hidding.
Banette: It's in the mt pyre but inside the building/tower (no the mountain path), in the top.
Again, the game really needs an in game guide to find all that is new, there is actually a new pokemon (from future generations) you can get in the first zone, how does he expect me to know that? I'm not even sure if the game is always there or i if had to progress to make it appear.