Why is there two shades of stone on the Washington Monument?

The two different shades of stone on the Washington Monument are the result of a change in the quarry from which the stone was quarried. The lower 150 feet of the monument is made of gneiss, a dark gray stone that was quarried from a quarry in Aquia Creek, Virginia. The remaining 350 feet of the monument is made of marble, a white stone that was quarried from a quarry in Cockeysville, Maryland.

The original plan was to build the entire monument out of gneiss, but the quarry in Aquia Creek was exhausted before the monument was completed. As a result, the architects were forced to switch to marble for the upper portion of the monument. The two different shades of stone are clearly visible from the ground, and they create a striking contrast that adds to the monument's beauty.