flint
A dense, fine-grained stone; a form of silica; naturally occurs in the form of nodules; usually gray, brown, black, or otherwise dark in color, but nodules and other chunks tend to weather white or light shades from the surface inward. Broken “flints,” as the nodules are called, are used in cobble size, either whole or split (knapped) in mortared walls, especially in England. (Harris, 1977)