If held by a Blastoise, 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 Blastoisenite or remove a Blastoisenite 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.
Battle Tree (from Red or Blue after defeating him in a Single Challenge)
In the anime
Blastoisinite in the anime
A Blastoisinite belonging to Siebold appeared in Mega Evolution Special I, during the his battle with Alain. The Blastoisinite is worn around Blastoise's head.