One the page for Mega Diancie (via pokemon.com) it says that to obtain Mega Diancie, you need to trade your Diancie from X/Y over to OR/AS. That means that the mega stone can't be transferred back to X/Y...
"Yo Dr Insano, I'm really happy for u and I'm a let you finish, but womens rights was one of the best trolls ever." - Valkskorn
exactly. currently all the mega stones are tradeable, and have you ever known of a pokemon game from the same generation that isn;t compatible with another one? (i.e Platinum and Heart Gold) Honestly i believe they'll just release a patch for X &Y, cause it looks like they could, and seeing how much they've done so far, and what they could do, it would be utterly ridiculous if X&Y aren't compatible with ORAS in such a way
ORAS will be able to interact with XY in the same ways FRLG can interact with RSE and HGSS can interact with DPPt. It goes both ways. The only time you can send things one-way is when a new generation is introduced, and trust me. ORAS aren't going to be the first 7th gen games.
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