Very fine-grained quartz , a silica mineral with minor impurities.
Flint is gray to black and nearly opaque (translucent brown in thin splinters). Opaque, dull, whitish to pale brown or gray specimens are called simply chert. Chert and flint provided the main source of tools and weapons for Stone Age people. Flint is used today as an abrasive agent on sandpapers and in mills that grind raw materials for the ceramic and paint industries. Considerable amounts of chert are also used in road construction and as concrete aggregate. Some chert takes an excellent polish and is used as semiprecious jewelry. See also siliceous rock .