Flint, a form of quartz. It consists of silica (SiO2) with traces of iron, aluminum, or calcium. Flint is a variety of cryptocrystalline quartz; that is, its crystals are too small to be seen without the aid of a microscope. It has a somewhat waxy or greasy appearance, varies in color from gray through brown to black, and is more than 2 12 times as heavy as water. It fractures readily to a thin, sharp edge somewhat like that of an oyster or clam shell. Flint is extremely hard and gives off sparks when struck with pyrites or steel. Flint occurs as irregular lumps called nodules, usually within masses of limestone or chalk, such as the chalk cliffs of Dover, in southern England.

There is no clear distinction between flint and chert. They have the same composition and properties, except that chert is found in bedded deposits in addition to the nodular form and it is generally lighter in color.

Because it fractures to a sharp edge, flint was widely used by prehistoric humans as a material for tools and weapons such as knives, scrapers, and arrowheads. Until friction matches came into use in the 1830's, a chief means of making fire was to ignite tinder with sparks struck from flint. The flintlock, used on early firearms, depended on sparks from flint for its firing. The flints used in cigarette lighters are made not of flint but of a cerium-iron alloy.