So I was wondering, since X and Y have the data for the Latis' Megastones. Can the Latasite be traded between the them and oras? I know the answer is probabily a big, fat no but, hey, it's worth a try.
Both Fondant and Stunnedshark are correct. The megastone for Latias and Latios cannot be transfered over from ORAS to X/Y but they are indeed encoded into the X/Y game. The only way to get the megastone in X/Y is through cheats :)
All ORAS introduced Megas are not compatible with X&Y, and neither are their mega stones.
Serebii - ORAS/ XY compatibility
I once traded my Latios with Latiosite to my friend with X and Y, and he tried using the Mega Stone, and it worked! The Mega Stones are coded into the data of the game, so it is usable in X and Y.