On December 6, 1884, workers placed the 3,300 pound marble capstone on the Washington Monument, and topped it with a nine-inch pyramid of cast aluminum, completing construction of the 555-foot Egyptian obelisk.
The Library of Congress has more. That’s their photo (click to view larger version). Looking at the photo, I’m wondering where was the photographer standing?
[The photo is not from the completion in 1884.]
At the time, that hunk of aluminum was the largest piece of aluminum in the world!