Why do people place rocks on top of gravestones? Why not personalize you own special rock?

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.