Chert is a hard, fine-grained sedimentary rock composed primarily of microcrystalline quartz (silica). It often forms in marine environments as nodules or layers within limestone or chalk and can also be a product of diagenetic processes. Chert is significant in geology for its use in reconstructing past marine environments and as a source of ancient tool-making material.
Reference: Hein, J. R., & Parrish, J. T. (1987). “Distribution of Siliceous Deposits in Space and Time.” Paleogeography, Paleoclimatology, Paleoecology, 60(1-2), 1-7.