I just returned from a visit with my Japanese friend in Hachinohe. While we were walking on a local beach, my granddaughter found a small, flat, oblong, smooth stone with kanji on both sides. It was in the edge of the water. When she showed it to my Japanese friend, she said "OH, you must put it back. It's for a dead person". I have read nothing in my studies about this custom. Could you enlighten me about this?