BoulderBadge (Japanese: グレーバッジ, Gray Badge) (hex:15) is an unavailable inventory item in Pokémon Red, Blue, Yellow.
In the final game the BoulderBadge is added to the player's Trainer Card and not the inventory, however BoulderBadge is a list entry for the badge man in Cerulean City.
In Pokémon Red, Blue, Yellow, when BoulderBadge is used outside of battle on the 'overworld' (02) sound bank, it will throw some bait (like in the Safari Zone) and play an isolated channel of a tune (possibly the 'follow me' theme). If used on the 'dungeon' (1F) sound bank it will instead play an isolated channel of the title screen music for the relevant game (which differs between Red/Blue and Yellow). This bait will halve the catch rate of the foe.