If held by a Garchomp, the Garchompite allows it to Mega Evolve into Mega Garchomp 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 Garchompite from Garchomp.
Outside of battle
If held by a Garchomp, the Garchompite allows it to Mega Evolve into Mega Garchomp when it performs a Spectacular Talent during a Pokémon Contest Spectacular if its Trainer has a Key Stone. Garchomp will remain a Mega Garchomp until the Talent Round ends.
A Garchompite was first seen in Mega Evolution Special I, belonging to Remo. His Garchomp, Garchoo, wears the Mega Stone around his neck. The same Garchompite made a brief reappearance in Analysis Versus Passion!.