"The United States Capitol dome is the dome situated above the United States Capitol which reaches upwards to 288 feet (88 m) in height and 96 feet (29 m) in diameter. The dome was designed by Thomas U. Walter, the fourth Architect of the Capitol, and constructed between 1855 and 1866 at a cost of $1,047,291 (equivalent to $232 million in 2015).
The dome is not stone, but cast iron carefully painted to appear to be made of the same stone as the main capitol building. It is actually two domes, one inside the other, and the total weight is 14.1 million pounds (6,400 t). The iron for the dome was cast by the foundry of Janes, Fowler, Kirtland & Company, owned by Adrian Janes in the Bronx, New York. (Wikipedia)"