Flint is the dark variety of chert that contains organic matter. It was a favored material of prehistoric humans, who used it (and chert in general) to make tools and weapons, because it would chip into sharp edges. Later, it was discovered that flint gave off a spark when struck against some hard metals and could be used to start fires. Many geologists and archaeologists suggest that the term flint be discarded or used only to identify artifacts of prehistoric humans.

It is a hard, extremely dense, dull to semiglossy sedimentary rock (microcrystalline to cryptocrystalline quartz). Chert consists predominantly of silica, with occasional impurities such as calcite, iron oxides, clay minerals, and the organic remains of marine organisms made of silica, amounting to about 10 percent. Because it is so finely crystalline, a characteristic of chert is its conchoidal fracture; thus, it breaks like glass into smooth, curved flakes. It may be white or one of various shades of gray, green, pink, red, yellow, brown, or black. Chert (Flint), occurs principally as nodules or concretions in limestone, dolomites, and chalk beds. Sometimes, however, it forms a bedded or layered deposit or a thin wedgelike discontinuous layer; such beds are commonly associated with volcanic deposition. Some are made up largely of spines and shells of silica secreted by microscopic organisms such as diatoms, radiolarians, and sponges,or of their partly dissolved and reprecipitated remains. Other cherts are of inorganic origin. Some precipitated around hot springs rich in silica, others formed when silica bearing solutions replaced wood, limestone, shale, or other materials, and some are associated with volcanic activity.