From the Super Mario Wiki
The Stone is an item solely in the NES release of Mario is Missing!. It is a single rock that laid on the highest point of Montreal's Mount Royal. When Koopa Troopas invade the city, they steal this rock, and the entire mountain is closed to the public until it is returned. Luigi eventually enters the city and takes it back, then asks the locals about it as he does not know where it belongs. All of them recognize the rock as belonging to the mountain. After gathering enough information, Luigi takes it to Mount Royal's information booth and answers a question to prove the item's legitimacy, whereupon the mountain is reopened.