This chunk of white marble is roughly the size and thickness of a deck of playing cards, and if the small metal plaque on it is to be believed, it was once a part of the White House until it was removed in 1950 when Harry Truman was president. I have no idea how to have it validated, but it only cost me three bucks so I'm not too worried about it. It's definitely one of the strangest things I've ever found for sale at a flea market.
