The stone chest is found the House on the Hill. Upon trying to search the chest, the message "It looks like the hole in the front of this locked chest exactly the size of a numulite."
If carrying numulite, the following message appears after searching: "Insert a numulite into the hole?" Upon selecting to insert a numulite, the message "You place a numulite into the hole, it rattles around and a mechanism clicks a few times, and the lid opens ever so slightly." appears.
After, this message appears: "You notice a scratched number 100 near the hole. Perhaps you should feed in more numulites?" The option to do so appears next.
Inserting the full 100 gives the chance to deal damage, have nothing happen, or to receive one of the notes to be added to Fossil island note book.