Considering to the question of Is it illegal to take a piece of the Berlin Wall, I am interested in knowing what other historical landmarks or sites one can legally and safely take home a piece.
Specifically, how would one determine if an arbitrary landmark has a legal route for acquiring a piece? In the case of the linked question, it was sheer luck that someone who knew a workaround frequents Travel.SE and saw the question.
Consider the avid traveler who might start a personal pebble-sized museum of historical places he had visited. The linked question suggests that he could add the Berlin Wall to his museum. What else could he legally add? The Great Wall of China? The Great Sphinx of Giza?