A Beanstone is a collectible in Mario & Luigi: Superstar Saga. It reappears in the remake. Beanstones only appear during the sidequest. After Bowletta invades the Beanbean Kingdom, the sidequest is unavailable.
It is a blue gemstone tha flashes different colors. In Mario & Luigi: Superstar Saga + Bowser's Minions, it appears to flash in diffent colors but the Beanstone is green in the inventory.