As visitors walk through Arlington National Cemetery en route to the Tomb of the Unknowns or John F. Kennedy’s grave, they often ask why rocks are atop tombstones.
Simply, it’s a Jewish tradition. Rocks serve as a reminder of one’s visit to a grave much like Christians leave flowers.
The tradition dates back to Jacob’s sons placing rocks atop their mother Rachel’s grave. It also serves as a cover to a grave in sandy climates where winds may expose the buried to animal predators.
The rock tradition has spread. Some Muslims do it and I’ve seen rocks on markers with Christian symbols. Colored beads are also used so it appears a widening tradition.