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What if Diancie could Diancie could make mega stones? In the movie, Diancie made a diamond for Bonnie, that also made it capable of mega evolution. Now, what if all Diancie could do this? You could transfer it over and have it make mega stones for you. But, you can't just have Diancie make mega stones out of nothing, it needs something to work with.
But, what could be used? The gems. Have the gems be used as crafting material for mega stones, and an everstone for the key stone. All you need is a Diancie in your party, a pokemon capable of mega evolution, and gems that correspond with the mega evolution you want. You go to your party, select Diancie, and select "Special"(an exclusive technique only that pokemon can do).
Choose the mon you want to make a mega stone for, and choose the gems you need. Simple as that.