Formstone is a type of stucco commonly applied to brick rowhouses in many East Coast urban areas in the United States, although it is most strongly associated with Baltimore. Formstone is commonly colored and shaped on the building to imitate various forms of masonry compound, creating the trompe l'oeil appearance of rock.
Formstone was patented by Albert Knight of Baltimore in 1937, although a similar product named Permastone had been invented in Columbus, Ohio, eight years prior. The name Formstone was actually a brand name used by Knight. Permastone, Fieldstone, Dixie Stone, and Stone of Ages were names used for a product similar to Knight's Formstone, particularly in other cities.
Formstone is applied in three layers, anchored by a metal lath attached to the underlying brick. The last layer contains the coloration used to imitate stone and is textured using waxed paper and an aluminum roller.
Formstone was used widely in Baltimore city. According to a local historian, formstone was widely applied because it covered the porous and leaky bricks used in working-class neighborhoods. Film director and Baltimore native John Waters described formstone as "the polyester of brick."
Formstone, described as "[a]n odd architectural fad" by urban design critic John King, appeared in San Francisco in the 1930s and '40s. While not particularly common, it is still found around the city.