Chert is a hard and compact sedimentary rock, consisting dominantly of very small quartz crystals. Chert occurs mostly in carbonate sedimentary rocks as nodules or layers. Flint is a dark-colored variety of chert.
Flint is gray to black and nearly opaque (translucent brown in thin splinters) because of included carbonaceous matter. Opaque, dull, whitish to pale-brown or gray specimens are simply called chert; the light colour and opacity are caused by abundant, extremely minute inclusions of water or air. The physical properties are those of quartz.
Chert and flint provided the main source of tools and weapons for Stone Age man. The uniform fine grain, brittleness, and conchoidal fracture made it relatively easy to shape arrowheads by flaking off chips, and the edges produced were quite sharp. Quarrying and manufacture of flint weapons were among humankind’s earliest business ventures, and it is sometimes possible to trace ancient trade routes by knowing where a particular type of flint was obtained. From the 17th through the early 19th century, flints again found extensive military use in flintlock rifles. Crushed flint is still used as the abrasive agent on sandpapers for the finishing of wood and leather. In addition, flint pebbles are used in mills that grind raw materials for the ceramic and paint industries; the use of flint pebbles instead of steel balls as a grinding agent is desirable in order to avoid contaminating the product with iron. Considerable amounts of chert are also used in road construction and as concrete aggregate. Some chert takes an excellent polish and serves as semiprecious jewelry.
Texture – non-clastic.
Grain size – cryptocrystalline, cannot be seen except under very high magnification.
Hardness – hard.
Colour – all colours, dependent on impurities present when precipitated.
Clasts – none.
Other features – smooth to touch, glassy, exhibits conchoidal fracture.
Uses – mainly decorative; ancient cultures used it for cutting tools, arrow heads etc.
Sources:Encyclopedia britannica; Sandatlas