chert

Chert is a hard, fine-grained sedimentary rock consisting of microcrystalline to cryptocrystalline quartz (SiO2). It typically occurs as nodules within carbonate rocks such as limestone or dolostone, or as layered beds in silica-rich sediments. Chert has a hardness of about 7 on the Mohs scale and a conchoidal fracture, making it capable of sharp edges.

Formation: Chert forms from the diagenetic precipitation of silica from groundwater or pore waters, often replacing calcite in carbonate rocks or precipitating in siliceous sediments. It can originate from biogenic silica derived from diatoms, radiolarians, or sponge spicules, as well as from chemical precipitation of silica.

Color and textures: Chert ranges in color from gray to brown, red, yellow, or green, depending on impurities such as iron oxides. It is typically dull or waxy in luster and can be translucent in thin fragments. The rock commonly occurs as nodules or beds; flint is the name given to chert nodules typically found in chalk in Europe, whereas jasper refers to opaque red-brown varieties.

Uses and significance: Throughout prehistory, chert was valued for its ability to be flaked into sharp tools and weapons. In modern geology and industry, chert is used as a source of silica and as a decorative or building stone in some regions.

Occurrence: Chert is widespread in sedimentary sequences worldwide and can form in marine or lacustrine settings. It is typically resistant to weathering and can form hardened outcrops or subsurface beds.