Clay ironstone is a fine-grained sedimentary rock frequently described as a concretionary form of siderite. Essentially, the rock is composed of the carbonate or oxide of iron and clay or sand. The color it displays is usually gray, but the outer layers of the material are often brown from oxidation. Relatively heavy, the rock is most often found in the form of nodules, though it also occurs in larger masses that may be flat, round, or lens-like in shape. Many times deposits of the rock are found overlying coal measures. In an economical sense, clay ironstone is generally considered an ore of iron that is of minor significance. However, in Great Britain this rock has been mined considerably and is of more importance than in most other parts of the world