If held by a Gyarados (or is present in the player's Bag)PE, allows it to Mega Evolve into Mega Gyarados if its Trainer has a Key Stone. Each Trainer can only Mega Evolve one Pokémon per battle.
Unaffected by item negating effects (such as Embargo and Klutz). Item manipulating effects (such as Trick and Fling) cannot give Gyarados a Gyaradosite or remove a Gyaradosite from Gyarados.
Outside of battle
If held by a Gyarados, allows it to Mega Evolve into Mega Gyarados when it performs a Spectacular Talent during a Pokémon Contest Spectacular while its Trainer has a Key Stone. Gyarados will remain a Mega Gyarados until the Talent Round ends.
Another Gyaradosite appeared in When Regions Collide!, where it was held by Misty's Gyarados. It didn't appear to be equipped on Gyarados, but rather, it appeared on its head fins right before Mega Evolving.
In the manga
A Gyaradosite in Pokémon Adventures
In the Pokémon Adventures manga
Lysandre owns a Gyaradosite, which first appeared in Chespin Sows the Seeds of Change. After Celosia stole Korrina's Key Stone, it was fashioned into a Mega Ring for Lysandre, allowing him to use it and the Gyaradosite to Mega Evolve his Gyarados. Eventually, X stole the Key Stone back from him.
