If held by an Aggron, the Aggronite allows it to Mega Evolve into Mega Aggron 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 Aggronite from Aggron.
Outside of battle
If held by an Aggron, the Aggronite allows it to Mega Evolve into Mega Aggron when it performs a Spectacular Talent during a Pokémon Contest Spectacular if its Trainer has a Key Stone. Aggron will remain a Mega Aggron until the Talent Round ends.
Erases Pokémon and disruptions around three 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 3.
An Aggronite appeared in The Prism Between Light and Darkness!, under the ownership of Team Rocket Elite Officer Gozu. The Aggronite didn't appear to be equipped on Aggron, but rather, it appeared on its head right before Mega Evolving.
Merchandise
The Aggronite has been featured in several officially licensed merchandise.
Mega Ring System Aggronite
Aggron, Mega Aggron & Aggronite metal charm set
