Rabbi Andrew Straus offers the following explanation:
Ritual is a way of expressing our emotions and spiritual needs. We need physical acts to express these things for us, to make them concrete.
Placing a stone on a grave does just that. It works in several ways:
1) It is a sign to others who come to the grave when I am not there that they and I are not the only ones who remember. The stones I see on the grave when I come are a reminder to me that others have come to visit the grave. My loved one is remembered by many others and his/her life continues to have an impact on others, even if I do not see them.
2) When I pick up the stone it sends a message to me. I can still feel my loved one. I can still touch and be touched by him/her. I can still feel the impact that has been made on my life. Their life, love, teachings, values, and morals still make an impression on me. When I put the stone down, it is a reminder to me that I can no longer take this person with me physically. I can only take him/her with me in my heart and my mind and the actions I do because he/she taught me to do them. Their values, morals, ideals live on and continue to impress me - just as the stone has made an impression on my hands - so too their life has made an impression on me that continues.
Coins are to pay the passage to the other side, and the keys are to unlock the gates of Heaven.
Why am I writing this? Because this week I visited the Holocaust memorial in Boston. Incredibly powerful and brilliant use of limited public space. One has to be there to appreciate the sense of place, of course, but briefly--what do you do when you are running out of horizontal space but have the awesome responsibility of listing six million concentration camp prisoner numbers, incorporating quotes and historical facts into a small garden space that also must contain a path for visitors to follow? You build vertically. You create delicate towers of illuminated etched glass over industrial grating. Grating that hisses quietly as steam escapes from below, a visceral reminder of gas chambers, ovens and the stifling deaths of many, many souls.
Memorials and their design are strange creatures. For its small size and crowded, public location, the experience was solitary, quiet and heavy. I walked there during the day. I cannot imagine what walking through those towers at night would feel like.
Upon leaving, I placed a penny on the carved granite wall. I didn't have a stone, but wanted to pay my respects, to leave a sign of remembrance and honor. I was glad to be alone. I wrote down a few quotes and facts from the memorial, I was so moved by them.
"When my parents were sent off to the camp, I gave my good shoes to my father because I thought he'd need them if he did physical labor. When I saw my mother for the last time, I hugged her and said I hoped she didn't have to work too hard. I never dreamed they'd be dead within such a short time of their departure." ~ Jack Polak, Holocaust survivor.
"Ilse, a childhood friend of mine, once found a raspberry in the camp and carried it in her pocket all day to present to me that night on a leaf. Imagine a world in which your entire possession is one raspberry and you give it to your friend." ~ Gerda Wiessman Klein, Holocaust survivor.
"After the German army invaded Denmark, the Danish people mobilized to ferry 7,800 Jews to safety in neutral Sweden. At the end of the war, 99% of Denmark's Jews were still alive."