If held by an Ampharos, the Ampharosite allows it to Mega Evolve into Mega Ampharos 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 an Ampharosite from Ampharos.
Outside of battle
If held by an Ampharos, the Ampharosite allows it to Mega Evolve into Mega Ampharos when it performs a Spectacular Talent during a Pokémon Contest Spectacular if its Trainer has a Key Stone. Ampharos will remain a Mega Ampharos until the Talent Round ends.
An uncut Ampharosite appeared in a flashback in Mega Evolution Special II, having been found by Lysandre before Alain could claim it. Alain battled Lysandre for it, but lost.
Ampharosite in the Mega Ring System Mega Bracelet Plus Black Metallic set
Ampharos, Mega Ampharos & Ampharosite metal charm set
Poncho-wearing Pikachu Pikachu in Mega Ampharos poncho figure
Mega Ampharos & Ampharosite keychain
