Gruenke iron headstone in section D-South of Prospect Hill Cemetery in Washington, D.C., in the United States in 2014. Prospect Hill Cemetery was added to the Register of Historic Places of the District of Columbia (a listing of historic sites maintained by the city) in 2005. The headstone is unique because it is made of iron, and painted white. Iron grave markers became fashionable in the late 1800s due to the widespread availability of iron from improved manufacturing processes, and because it was considered to be a long-lasting material.