Question

How do you get Gardevoirite on X/Y?

Answer 1

You can visit Lumiose City after beating the E4 once and trade any Pokémon for Diantha's Ralts. This Ralts will be holding Gardevoirite.

Answer 2

Post game, in Lumiose City's Soleil Cafe, Diantha will be there needing to trade a Ralts with a Gardevoirite as an item.

Comments

Please don't answer an already answered question unless your answer adds a lot of new information.

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This question doesn't add a lot of information, but it does add new information that is useful. The original answer just said that Diantha was in Lumiose City, but this answer says where in Lumiose City it is.