Cap of the Monument (Reproduction)
The builders searched for appropriate metal for the cap that would not tarnish and would act as a lightening rod. They chose one of the rarest metals of the time -- aluminum.
Efforts were made to build monuments to Washington during his lifetime, but he was not enthused about the idea. As President, he insisted that public monies should be used for other more important things.
The obelisk that towers 555' 5/8" over Washington DC had the cornerstone laid on July 4, 1848. Construction continued for 10 years. It was interrupted by the War between the States. When construction resumed in 1878, the marble could not be matched and you can see a distinct color shift about a third of the way up the monument. The capstone was placed December 6, 1888.
When you visit, be sure to stand close to the doors of the elevator on the way down. There are carved commemorative plaques inside the monument and they stop to let you see some of them on the way down. These plaques were sold as fundraisers for the completion of the monument.
For the specifications of the monument: http://www.nps.gov/archive/nama/mortar/mortar.htm