A moon stone is an item in the "Pokémon" game series that transforms certain kinds of pokémon into different species, depending upon each pokémon's lineage. For example, a Jigglypuff exposed to a moon stone evolves into a Wigglytuff, but a Clefairy exposed to one turns into a Clefable.
The moon stone has appeared in all seven generations of the main "Pokémon" games, as well as appearing in the spin-off "PokéWalker." In the first game, the item is found in a cave system called Mt. Moon. In later games, it can occasionally be found as a held item when capturing certain wild pokémon.