Chert
A fine-grained rock, often called "flint." The terms “microcrystalline” or “cryptocrystalline” are used to describe the size of the silica grains in chert, which are not visible to the eye (Luedtke 1992). It ranges greatly in color. Gray, black, and white variations are common, but it can also be shades of green, red, brown, or other colors. Heat treating chert often results in stone that is waxier, darker, and/or redder in appearance. Chert occurs in thin beds or as irregular nodules in sedimentary carbonate formations such as limestone. Shriver chert in western Maryland is an example of bedded chert. Chert cobbles that have washed out from their parent formations can be found along waterways throughout Maryland. Because flowing water can move small rocks farther than big ones, the largest cobbles generally are found closer to river headwaters.
