Bottle Capin Pokémon Polished Crystal

A shiny, rare bottle cap. These are used for Hyper Training. Take one to the Hyper Trainer and they'll max out one of your Pokémon's IVs at level 100. I'd say these are mainly useful for legendaries and shinies - situations where you can't just breed for better IVs. If you manage to find a Gold Bottle Cap, that'll max all IVs at once, which is pretty nice.