give your painted friendship stone a unique id, place it somewhere and follow on a map how far it travels as others find it and hide it again.
paint a stone, add it in the app with a photo and a name, and get a short, readable id, for example h7f2k, to write on the back.
leave the stone somewhere and report the location. the stone appears on the map and waits for a finder.
whoever finds the stone enters the id and reports the find. the creator gets notified and sees the new stop on the stone's travel page, and the finder hides it somewhere new.