From Bulbapedia, the community-driven Pokémon encyclopedia.
The Wish Stone (Japanese: うたごえのいし Sing Stone) is an item that only exists in Pokémon Mystery Dungeon: Red Rescue Team and Blue Rescue Team. It is obtained by rescuing Medicham of Team Meanies in Wish Cave, but another Wish Stone can be found on Wish Cave's 50F.
The Wish Stone has only one use. At the end of Wish Cave, the player will battle Jirachi. Upon being defeated, Jirachi will automatically ask to join the rescue team. However, if the player possesses the Wish Stone and turns down Jirachi's offer to join the team, Jirachi will offer to grant the player a wish.
If the player defeats Jirachi and has the Wish Stone, Jirachi allows the player to choose one wish from the following list.
Lots of Money
Jirachi fills the entire room with Poké, and allows the player to roam around the room and pick it all up.
Lots of Items
Jirachi fills the entire room with miscellaneous items, and allows the player to roam around the room and collect them.
A Friend Area
Jirachi gives the player a new Friend Area out of the Friend Areas that have not yet been obtained.
Jirachi randomly selects and gives to the player four of the following items; Calcium, Protein, Zinc, Iron, Joy Seed, Sitrus Berry, Life Seed.
Jirachi will send a non-legendary Pokémon that has not been recruited to show up in front of the base and be recruited.