Question

Can some Mega Stones only be obtained after beating the E4?

Answer 1

Yes. Absolite, Medichamite, Gardevoirite, Houndoomite, Manectite, Tyranitarite, and Mewtwite are found after the Elite Four.

Source

Comments

*Mewtwonite

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you forgot Aggronite.

And some Mega Stones are exclusive to either X or Y.

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Mawilite, Gengarite, Gyradosite, Banetteite, Scizorite, and the Mega stones for the two starters you didn't pick.