Manganese nodules are rock concretions on the sea bottom formed of concentric layers of iron and manganese hydroxides around a core. Nodules vary in size from tiny particles visible only under a microscope to large pellets more than 20 cm across. However, most nodules are between 5 and 10 cm in diameter, about the size of potatoes. Their surface is generally smooth, sometimes rough, mammilated (knobby) or otherwise irregular. The bottom, buried in sediment, is generally rougher than the top. This earthy brown specimen is derived from a hot springs vent associated with volcanic activity on the deep sea bottom. Light weight for its size, it weighs only 27 grams. Ex. Tarnowski Collection.