The cornerstone for the Washington Monument was laid in 1848. Still the tallest structure in D.C., at the time of its completion, it was the tallest building in the world. Construction took several decades and was interrupted due to funding issues and during the Civil War.
The Monument was officially dedicated on February 21, 1885. It is still the tallest stone structure in the world.
 
Inscribed on the east face of the capstone are the Latin words “Laus Deo” ~ “Praise be to God.” Amen.
 
 

Sources:
The Washington Monument and Laus Deo-I – THE PYRAMIDION ~ at America’s Christian History website
Washington Monument ~ Wikipedia