"One variety of the mysterious Mega Stones. Have Metalynx hold it, and this stone will enable it to Mega Evolve."
Metalynxite can be bought from the Bealbeach Department Store on Wednesdays for 50,000 or obtained for free if you have a Metalynx in your party after defeating gym leader Vaeryn and you select that Metalynx after being asked what Pokémon you would like to be able to Mega Evolve.