Flint (or flintstone) is a hard, sedimentary cryptocrystalline form of the mineral quartz, categorized as a variety of chert. It occurs chiefly as nodules and masses in sedimentary rocks, such as chalks and limestones. Inside the nodule, flint is usually dark grey, black, green, white, or brown in colour, and often has a glassy or waxy appearance. A thin layer on the outside of the nodules is usually different in colour, typically white, and rough in texture. From a petrological point of view, "flint" refers specifically to the form of chert which occurs in chalk or marly limestone. Similarly, "common chert" (sometimes referred to simply as "chert") occurs in limestone.
The exact mode of formation of flint is not yet clear but it is thought that it occurs as a result of chemical changes in compressed sedimentary rock formations, during the process of diagenesis. One hypothesis is that a gelatinous material fills cavities in the sediment, such as holes bored by crustaceans or molluscs and that this becomes silicified. This theory certainly explains the complex shapes of flint nodules that are found. The source of dissolved silica in the porous media could arise from the spicules of silicious sponges.
Flint was used for the manufacture of flint tools during the Stone Age, as it splits into thin, sharp splinters called flakes or blades (depending on the shape) when struck by another hard object (such as a hammerstone made of another material). This process is referred to as knapping.
In Europe, some of the best toolmaking flint has come from Belgium (Obourg, flint mines of Spiennes), the coastal chalks of the English Channel, the Paris Basin, Thy in Jutland (flint mine at Hov), the Sennonian deposits of Rügen, Grimes Graves in England and the Jurassic deposits of the Kraków area in Poland. Flint mining is attested since the Palaeolithic, but became more common since the Neolithic (Michelsberg culture, Funnelbeaker culture).
Even so, many lighters still use a variant of the ancient technology as the ignition source for their primary fuel, due to the ease with which those materials generate a sufficiently hot spark. Ferrocerium now replaces the steel and has been miniaturized and integrated into such lighters. The ferrocerium used in these lighters, while sometimes called "flint", has the opposite role than the flint in true flint-and-steel. Ferrocerium is much softer than modern steel and can be scraped with knife with minimal damage to the knife. In lighters, a small steel wheel is turned by the user rotating it with the thumb, creating a high-temperature spark, which ignites the lighters' primary fuel source (which is either present, such as in liquid fuel lighters, or which is also released by the same thumb motion, such as in butane-gas lighters), which then produces a small flame that burns until extinguished. Common examples include the disposable Bic lighter and the reusable Zippo lighter. The small ferrocerium cylinder in Zippo's spark producing mechanism is eventually consumed, and Zippo and other companies (e.g., Ronson) sell replacements that are marketed as "flints," even though they are ferrocerium. Such mechanisms are more popular in liquid fuel lighters than in butane-gas lighters, which more commonly have a piezoelectric ignition system.
A later major use was to create the spark that would ignite the powder that would fire a ball or bullet from a flintlock firearm. While the military use of a flintlock declined after the British military generally applied the percussion cap on their muskets in 1842, it is still popular to use the flintlock as a hunting rifle during special muzzleloader seasons or general rifle seasons in several states in the US.
In the UK, flint pebbles were traditionally an important raw material for clay-based ceramic bodies. After calcination to remove organic impurities and induce certain physical reactions, and milling to fine particle size, flint was added as a filler to pottery bodies. However, flint is no longer used and has been replaced by quartz as is used in other countries. Because of this historical use, the word "flint" is used by US potters to refer to siliceous materials which are not flint.