Why do Jews put small stones on tombstones when visiting the cemetery?

The earliest graves may have been covered by a mound of stones. Though we erect tombstones today, a stone or pebble placed on a head or footstone reminds us of those first humble gravesites. This simple act has come to be a great sign of respect of our deceased loved ones.
It is come to signify that the grave has recently been visited and that the deceased have not been forgotten. To make this simple ritual even more meaningful, some bring a pebble or stone from their own garden to place on the tombstone, or select a brightly colored stone to place at the grave. Placing a stone on the grave of a loved one is a tradition that may be personalized to create meaning and bring comfort.