The two sections closely resembled each other at first, but time, wind, rain, and erosion have caused the marble sections to weather differently, thereby producing the difference in color. A third type of marble is also visible at the dividing line between the two main phases of construction.

The Washington Monument features a distinct "color line" about 150 feet up because its construction was interrupted for nearly 20 years. The bottom third was built between 1848 and 1854 using marble from a quarry in Texas, Maryland. However, construction was halted in 1856 due to a lack of funds and the onset of the American Civil War. When work finally resumed in 1876 after Congress authorized public funding, the original quarry was no longer available. Builders had to source marble from different quarries in Maryland and Massachusetts. While the new stones looked similar to the original ones when they were first installed, the different mineral compositions caused the two types of marble to weather and age differently over the last 140 years. This created the permanent "seam" seen today. This visible difference stands as a historical reminder of the political and financial controversies that nearly prevented the monument—once the world's tallest structure—from ever being completed.