I've transferred my Diancie to Omega Ruby, now how can I get its mega stone? :/
>Any Pokémon Center (upon entering after obtaining a fateful encounter Diancie)
Simply go into any Pokemon Center with a Diancie in your party, a few guys will talk to you and at the end of the conversation you will get a Diancite.
First, Get Diancie
Next, Enter any Pokemon center
Then, Talk to 2 people
Finally, They give you a Diancite
Hope I Helped! :D