Why do the survivors put rocks on Oskar Schindler's grave at the end of the story?

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.