File:Fossiliferous chert nodule with fossil sponge (Upper Cretaceous; Warminster, England, Britain) 2 (16799097866).jpg

Fossiliferous chert nodule with a fossil sponge from the Cretaceous of Britain. (public display, Field Museum of Natural History, Chicago, Illinois, USA)

Sedimentary rocks form by the solidification of loose sediments. Loose sediments become hard rocks by the processes of deposition, burial, compaction, dewatering, and cementation.

There are three categories of sedimentary rocks: 1) Siliciclastic sedimentary rocks form by the solidification of sediments produced by weathering & erosion of any previously existing rocks. 2) Biogenic sedimentary rocks form by the solidification of sediments that were once-living organisms (plants, animals, micro-organisms). 3) Chemical sedimentary rocks form by the solidification of sediments formed by inorganic chemical reactions. Most sedimentary rocks have a clastic texture, but some are crystalline.

Chert is a cryptocrystalline-textured, siliceous sedimentary rock. It is composed of quartz (SiO2). Traditionally, light-colored varieties were called “chert” by geologists, and dark-colored varieties were called “flint”. This arbitrary distinction is no longer preferred. “Flint” is now an archaeological term for chert that has been worked by early humans. "Flint" is generally perceived by rockhounds to be high-quality material (from a flint-knapper's point of view, apparently), whereas "chert" is perceived as low-quality material. Chert nodules in Cretaceous chalks of Britain are still called “flint” by some geologists. Chert meganodules at Flint Ridge, Ohio are called “flint” in the geologic literature.

Individual quartz crystals are incredibly small in cherts, and generally cannot be seen with normal microscopes. Chert comes close to having the physical properties of a glassy textured rock - it is very hard (H = 7), has conchoidal fracture (smooth & curved fracture surfaces), and has sharp broken edges.

Cherts vary in color. Common chert colors include whitish, grayish, brownish to dark gray, very dark blue, and black. Reds, yellows, and greens are sometimes present. Some cherts are complexly multicolored.

Some cherts form biogenically, but other cherts have a chemical origin. As a result, chert cannot be placed cleanly or neatly or unambiguously into a traditional sedimentary rock category (siliciclastic, biogenic, chemical).