Ironstone is a fine-grained, heavy and compact sedimentary rock. Its main components are the carbonate or oxide of iron, clay and/or sand. It can be thought of as a concretionary form of siderite. Ironstone also contains clay, and sometimes calcite and quartz. Appearance Freshly cleaved ironstone is usually grey. The brown external appearance is due to oxidation of its surface. Ironstone, being a sedimentary rock isn't always homogeneous, and can be found in a red and black banded form called t… (More on Ironstone)