nodular chert

Nodular chert is a type of chert, which is a very fine-grained quartz, a silica mineral. It is found as small to large, knotlike or fistlike clusters of quartz, chalcedony, and opal. These clusters are concentrated along or parallel to bedding planes within shallow-water marine carbonate rocks and pelagic limestones. Individual nodules can be ovoid or semispherical, and masses of chert often form an anastomosing network.1

Chert and flint are common in rocks of all ages and occur as individual nodules or layers of nodules in limestone or dolomite. They are hard and chemically resistant, often concentrating in residual soils as the surrounding carbonate rock weathers away.2