From the Super Mario Wiki
The Stone is an item in all versions of Mario is Missing!. It is one of the bricks that make up the Great Wall of China. It gets taken by a Koopa Troopa when a group of them invade Beijing, causing the entire structure to be closed off. Luigi eventually comes across it while exploring the city, and as he does not know where it belongs, he brings it to the city's citizens and asks them about it. Everyone that he meets recognize the generic brick as belonging to the Great Wall, and after gaining enough knowledge, he returns the item to the Wall's information booth, receiving $1225 as a reward.