The concept of taking a rock implies a sense of permanence, as previously alluded. There are many around the world who will take a rock or stone from a sacred place and keep it to represent something subjective that lies inside of them. By their nature, rocks are separate and disjointed, and there is nothing particularly about them at first glance that indicates special privilege. Yet, when they are taken with sentiment, they mean the world as they represent a part of the subjective that few, if any, will understand. The symbolic attachment they carry far outstrips any literal distinction the rocks once might have had. The placement of the rocks at Schindler's grave fits this in that it allows each of the survivors and actors that portray them to honor Oskar in a way that creates a personal tapestry of subjective emotions that recognize what he did. Notice how each of the rocks is different, indicating that Oskar Schindler meant so many things to so many different people. The arrangement of the rocks at the end, right before Liam Neeson places the roses in the center of the gravestone, helps to bring to light that those who were saved by Schindler helped to create a tapestry of a narrative, one that told the tale of unquestionable humanity at a point in time where humanity was unrecognizable. Complemented with the music from Perlman and Williams, the scene is quite powerful and moving as a tribute and a way to properly conclude a film that was equally compelling.
In the tradition of Jewish faith the placement of rocks on the grave is the equivalent of placing flowers in Western (namely, American) tradition. It is a very ancient tradition that dates back to a long time where people tried to protect decaying bodies from vultures and other animals. In fact, in the case of Oskar it became more than just a tradition- stones are non-living and last longer than flowers. Therefore the practice continued.