From the Super Mario Wiki
The Beanbean Brooch is a key item in Mario & Luigi: Superstar Saga. It has a green gem in its center with yellow spheres around the edge.
Lady Lima gives this item to Mario after he and Luigi defeat a berserk Queen Bean. The Queen was force-fed a Belly Blech worm by Cackletta, making her stronger but always angry. The only cure was the Chuckola Reserve, located in Chucklehuck Woods. The Beanbean Brooch is used to verify themselves to the guards in front of Chateau de Chucklehuck, which sits on the edge of Chucklehuck Woods.
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