Other Types

Argillite is a homogenous dark material that is metamorphosed from fine-grained stone like siltstone, mudstone, or shale.  It is commonly purple or black to gray, with gradations of color possible in a single stone.  Phenocrysts are sometimes present.  It weathers easily, which can change its color and create a porous or bumpy surface.  Argillite can be virtually indistinguishable from hornfels, which is produced from more-highly metamorphosed clay-rich stones (Andrefsky 1998).

Ferruginous sandstone, sometimes called ironstone, is formed when limonite – an iron oxide precipitate – cements sandstone grains together.  It typically occurs in “rusty” shades of red and brown.  High-quality ferruginous sandstone can be flaked like quartzite, and fluted points made from it have been found on the Eastern Shore (Ebright n.d.; Lowery 2002).