It was custom to write notes to the deceased and place them into the crevices of a headstone; similar to how writings are pushed into the Western Wall in Jerusalem. When there was no crevice, notes were weighted down with a stone. In time, the paper deteriorated or blew away leaving only the stone. Thus, some began to think that leaving a stone was customary. So now, before leaving the grave site, people place a small stone on the marker to indicate that they'ved visited the grave.