The Washington Monument is a hollow Egyptian style stone obelisk with a 500 - foot (152.4 m) tall column and a 55 - foot (16.8 m) tall pyramidion. Its walls are 15 feet (4.6 m) thick at its base and 1 ⁄ feet (0.46 m) thick at their top. The marble pyramidion has thin walls only 7 inches (18 cm) thick supported by six arches, two between opposite walls that cross at the center of the pyramidion and four smaller corner arches. The top of the pyramidion is a large marble capstone with a small aluminum pyramid at its apex with inscriptions on all four sides. The lowest 150 feet (45.7 m) of the walls, constructed during the first phase 1848 -- 54, are composed of a pile of bluestone gneiss rubble stones (not finished stones) held together by a large amount of mortar with a facade of semi-finished marble stones about 1 ⁄ feet (0.4 m) thick. The upper 350 feet (106.7 m) of the walls, constructed during the second phase 1880 -- 84, are composed of finished marble surface stones, half of which project into the walls, partially backed by finished granite stones.
what metal is on top of the washington monument
aluminum