At an unveiling ceremony a few weeks ago, the Rabbi gave a very interesting comment on the custom of leaving pebbles on the tombstones. He said that the death of a loved one "ligt vee a shtain of'n hartz" (like a rock on one's heart). Placing pebbles on the tombstone as a symbol of having visited the grave-site is, in essence, removing pieces of that "shtain" or rock.
