If held by a Heracross, allows it to Mega Evolve into Mega Heracross if its Trainer has a Key Stone. Each Trainer can only Mega Evolve one Pokémon per battle.
Unaffected by item negating effects (such as Embargo and Klutz). Item manipulating effects (such as Trick and Fling) cannot give Heracross a Heracronite or remove a Heracronite from Heracross.
Outside of battle
If held by a Heracross, allows it to Mega Evolve into Mega Heracross when it performs a Spectacular TalentORAS during a Pokémon Contest Spectacular while its Trainer has a Key Stone. Heracross will remain a Mega Heracross until the Talent Round ends.
Adds one more Mega Heracross to the left of the match. Actually converts the tiles directly to the left of all other Mega Heracross into a Mega Heracross. This effect can replace any disruption in the tile above and counts as erasing the content of the tile for damage, but does not affect black-cloud disruptions. Does nothing if the left tile is empty.