If held by a Camerupt, the Cameruptite allows it to Mega Evolve into Mega Camerupt 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 Cameruptite from Camerupt.
Outside of battle
If held by a Camerupt, the Cameruptite allows it to Mega Evolve into Mega Camerupt when it performs a Spectacular Talent during a Pokémon Contest Spectacular if its Trainer has a Key Stone. Camerupt will remain a Mega Camerupt until the Talent Round ends.
Erases Pokémon and disruptions around two spots you tap. Erases five tiles in a + shape surrounding each spot. Does not activate if the foe is defeated. Only gives 3 seconds to tap. Can erase fewer than 2.