A Beanstone is a collectible in Mario & Luigi: Superstar Saga and its Nintendo 3DS remake Mario & Luigi: Superstar Saga + Bowser's Minions. Beanstones only appear during the sidequest, so after Bowletta invades the Beanbean Kingdom, the sidequest becomes unavailable.
Description
It is a blue gemstone that flashes different colors. In Mario & Luigi: Superstar Saga + Bowser's Minions, it appears to flash in different colors, although the Beanstone is green in the inventory.
At the Beanbean Castle Town, Fava's brother, the town's oldest intellect, has a sidequest for Mario and Luigi to find him all of the ten Beanstones, which are buried underground around the town. In doing so, he rewards them the Bros. Rock Badge.