If held by a Sableye, the Sablenite allows it to Mega Evolve into Mega Sableye 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 Sablenite from Sableye.
Outside of battle
If held by a Sableye, the Sablenite allows it to Mega Evolve into Mega Sableye when it performs a Spectacular Talent during a Pokémon Contest Spectacular if its Trainer has a Key Stone. Sableye will remain a Mega Sableye until the Talent Round ends.
