You need two very rare stones. A Keystone, [he says, putting a hand on his mega-anchor, tapping the gem laid into the hilt.] And the corresponding gem for the Pokémon. Sharpedo can only mega evolve with Sharpedonite, Camerupt with Cameruptite, you get the idea.

[he shakes his head.]

They're... very hard to come by. Especially outside of Kalos.