Claim: The Washington Monument is harder than marble, granite, and bluestone gneiss.
Evidence: Located almost due east of the Reflecting Pool and the Lincoln Memorial , the monument , made of marble , granite , and bluestone gneiss , is both the world 's tallest stone structure and the world 's tallest obelisk , standing 554 ft tall according to the National Geodetic Survey -LRB- measured 2013 -- 14 -RRB- or 555 ft tall according to the National Park Service -LRB- measured 1884 -RRB-.