On a recent field investigation, I saw a stone in a cemetery where visitors had left small stones atop the name marker. Familiar with the folk magic custom of picking up a stone when passing a scary place or cemetery (dropping it without looking back once the boundary was passed) I was curious about this custom.
Best sources indicate that it is an ancient behavior often seen in Jewish cemeteries. It harks back to the ancient memorial marker or cairn building behaviors of ancient peoples. The placing of one or more small stones is a symbol of remembrance and a memorial to the life of the person buried there.
A modern development in this process of memorials could be seen all over the cemetery I visited. There, colorful pinwheels, stuffed animals, wind chimes, and other mementos adorned the markers of lost loved ones and friends. They were a poignant reminder of the tragedy of death and the grief that such loss leaves behind.