I have a Jewish cemetery near my home. What is the significance of placing rocks on the headstone?

Where other mourners place flowers, Jews lay pebbles. This custom has murky origins, but it is a sure way to leave evidence of a gravesite visit, one that will not wilt or blow away. Some attribute the custom to the ancient practice of covering a grave with stones. More stones would be added as time passed to keep the grave covered. Placing a pebble on the grave demonstrates care that extends beyond death.

http://www.mazornet.com/DeathAndMourning/OrthodoxV...

Traditionally Jews don't leave flowers, so we put a little pebble or stone on the grave as a show of respect and affection. That said, many Jews, myself included, DO take a little flowering plant to the cemetary and place it beside the grave. There are in fact very strict rules governing what is allowed in Jewish cemetaries, the idea being that all should be 'equal' in death; i.e. no show of ostentation is ever permitted.

Good question :)

http://www.whatjewsbelieve.org/

http://www.ajewwithaview.com/

Aidan's answer is good.

The stone indicates to others that the grave has been visited.

Count the stones and you will know how many visitors have been to that particular grave.