If you want to trade a Mega Stone from ORAS to XY that is coded into XY, then you can. It's a great and easy way to Mega Evolve some of your Pokemon during the story when they usually wouldn't be able to Mega Evolve until post game. All you have to do is make the Mega Stone the held item of a Pokemon and trade it to XY.
Unfortunately, you are unable to do these trades with Mega Stones that weren't introduced until ORAS.
Pokemon Bank also doesn't work, as any held items are sent back to the bag when depositing in Pokemon Bank.
Hope I could help!