Chert is 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 allowed in geology. “Flint” is exclusively an archaeological term in modern geologic technical terminology (well, it’s supposed to be). For rockhounds, “flint” is perceived as high-quality material (apparently, from a flint-knapper’s point of view), whereas “chert” is perceived as low-quality material (= not knappable). 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 perceived with normal microscopes. Chert comes somewhat close to having the physical properties of rocks having a glassy texture - 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 & neatly & unambiguously into one of the three sedimentary rock categories: siliciclastic, biogenic, chemical.
Chert (chipped chert) from the northern Arabian Desert. The concavities appearing on this chert surface have been attributed to natural “chipping” by intense, daytime solar heating. (public display, Field Museum of Natural History, Chicago, Illinois, USA)
Chert nodule (“flint”) from an Upper Cretaceous chalk succession at the White Cliffs of Dover, England. The black broken interior is the chert. The white material is chalk coating the chert nodule. (public display, Field Museum of Natural History, Chicago, Illinois, USA)
Chert nodules (“flint”) containing fossil sponges from an Upper Cretaceous chalk succession at Warminster, England. (public display, Field Museum of Natural History, Chicago, Illinois, USA)