What about Tom Riddle's rocks in Harry Potter and the Half Blood Prince?

In the movie, Harry Potter and the Half Blood Prince, Dumbledore visits Tom Riddle in the orphanage and you see several rocks on the window sill. Then, they reappear later on when Harry touches the Horcrux ring and has a flashback of memories or something. Is there a significance to those rocks? Or were they just put there for props?