Mega Stones are evolutionary stones that allow certain Pokémon to reach their final form for one-evolution families and second/true final form for two-evolution/branch-evolution families. These stones are worn in some sort of accessory for the Pokemon, but disappear upon Mega Evolving, reappearing after the battle they're involved in is over.
Each Mega Stone has a "-ite" or "-nite" on the end of it preceded by the related Pokemon's name (example: Lucario's Mega Stone is called Lucario-"nite", Blaziken's Mega Stone is Blaziken-"ite").
Certain Pokémon will be able to evolve into a Mega form with the use of a Mega Stone, a held object. However, this evolution will devolve into to their pre-evolved form at the end of the battle it's involved in.