A loosely defined term used for a sedimentary mineral segregation that may range in size from inches to many feet. Concretions are usually distinguished from the sedimentary matrix enclosing them by a difference in mineralogy, color, hardness, and weathering characteristics. Some concretions show definite sharp boundaries with the matrix, while others have gradational boundaries. Most concretions are composed dominantly of calcium carbonate, with or without an admixture of various amounts of silt, clay, or organic material. Less common are the clay-ironstone concretions characteristic of the Carboniferous coal measures in many parts of the world. The latter are mixtures of iron carbonate minerals and iron silicate minerals. Coal balls are calcareous concretions, found in or immediately above coal beds, in which there may be a high percentage of original plant organic matter, showing wonderfully preserved plant fossils in a noncompressed condition. Concretions are normally spherical or ellipsoidal; some are flattened to disklike shapes. Frequently a concretion is dumbbell-shaped, indicating that two separate concretionary centers have grown together.