If held by a Blastoise (or is present in the player's BagPE), the Blastoisinite allows it to Mega Evolve into Mega Blastoise if its Trainer has a Key Stone. A Trainer can only Mega Evolve one Pokémon per battle (or only have one Mega-Evolved Pokémon at any given momentZA).
As a Mega Stone, it is unaffected by item-negating effects (such as Embargo and Klutz). Item-manipulating effects (such as Trick and Fling) cannot give or remove a Blastoisinite from Blastoise.
Outside of battle
If held by a Blastoise, the Blastoisinite allows it to Mega Evolve into Mega Blastoise when it performs a Spectacular Talent during a Pokémon Contest Spectacular if its Trainer has a Key Stone. Blastoise will remain a Mega Blastoise until the Talent Round ends.
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.