http://www.myjewishlearning.com/lifecycle/Death/Burial_Mourning/TombstoneUnveiling.htm
Unveiling
It is customary for the grave marker to be put in place and for an unveiling ceremony to be held after the Kaddishperiod [11 months for parents and 30 days for other close relatives] is over, but no later than one year after the death. While many families wait until almost the full year has passed to do the unveiling, it may be done sooner; in Israel the stone is usually placed soon after sheloshim [the first 30 days of mourning].
The unveiling ceremony consists of the recitation of Psalms, a very brief eulogy encapsulating the most salient characteristics of the deceased, removing the cloth covering the headstone, the El Maleh Rahamim [God full of compassion--a prayer], and the Mourner's Kaddish[a prayer in praise of God recited by mourners]. Traditionally, Kaddish is not recited aloud if no minyan [quorum of 10]ispresent.
It is customary, before leaving the gravesite, to place a small stone on the marker to indicate that someone has visited the grave. This tradition may also reflect the biblical practice of marking the grave with a pile of stones. Or, it may be the end result of the custom of writing notes to the deceased and pushing them into crevices in the headstone just as notes are pushed into the Western Wall in Jerusalem. When no crevice could be found, the note was weighted down with a stone. In time, the paper disintegrated or blew away leaving only the stone. Thus, some began to think that the leaving of a stone was the custom... and so it became the custom.
That brought back SO many fond memories .... 
