If held by a Blastoise (or is present in the player's Bag)PE, allows it to Mega Evolve into Mega Blastoise 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 Blastoise a Blastoisinite or remove a Blastoisinite from Blastoise.
Outside of battle
If held by a Blastoise, allows it to Mega Evolve into Mega Blastoise when it performs a Spectacular Talent during a Pokémon Contest Spectacular while its Trainer has a Key Stone. Blastoise will remain a Mega Blastoise until the Talent Round ends.
The player can obtain the Blastoisinite by completing Mission Card 12.
The Blastoisinite was also available as a prize during the Mega Blastoise Competitive Stage. This Competitive Stage ran on the Nintendo 3DS version from April 6 to 12, 2015 and June 15 to 22, 2015. It ran on the Pokémon Shuffle Mobile from February 9 to February 15, 2016.
The Blastoisinite enables the following Mega Evolutions:
Erases all Pokémon within two spaces of the match. If more icons of this Pokémon are adjacent to the match, then for each additional icon, one more Pokémon is erased in that direction.
A Blastoisinite belonging to Siebold appeared in Mega Evolution Special I, where it was revealed during his battle with Alain. The Blastoisinite is worn around Blastoise's head.