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 color, and often has a glassy or waxy appearance. A thin layer on the outside of the nodules is usually different in color, 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. Certain types of flint, such as that from the south coast of England, contains trapped fossilised marine flora. Pieces of coral and vegitation have been found preserved like Amber inside the flint. Thin slices of the stone often reveal this effect.
Puzzling giant flint formations known as paramoudra and flint circles are found around Europe but especially in Norfolk, England on the beaches at Beeston Bump and West Runton.
Tools or cutting edges
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).
To ignite fire or gunpowder
When struck against steel, a flint edge will produce sparks. The hard flint edge shaves off a particle of the steel that, heated by the friction, burns with oxygen from the atmosphere and can ignite the proper tinder. Prior to the wide availability of steel, rocks of iron pyrites would be used along with the flint, in a similar (but more time-consuming) way. These methods are popular in woodcraft, bushcraft, and among those who wish to use traditional skills.
Striking sparks with flint and steel is not a particularly easy or convenient method to start a fire, although it is much easier than other primitive fire-making methods such as using a bow drill. As with most skills, practice improves results.
It is important to note that while flints may be used as above in fire-lighting, they should never be used in the construction of camp-fire hearths, or otherwise be exposed to heating by fire - as they can explode when hot.
A later, major use of flint and steel was in the Flintlock mechanism, used primarily in flintlock firearms, but also used on dedicated fire-starting tools. A piece of flint held in the jaws of a spring-loaded hammer, when released by a trigger, strikes a hinged piece of steel ("frizzen") at an angle, creating a shower of sparks and exposing a charge of priming powder. The sparks ignite the priming powder and that flame, in turn, ignites the main charge propelling the ball, bullet, or shot in the barrel. While the military use of the flintlock declined after the adoption of the percussion cap from the 1840s onward, the flintlock is still popular on hunting rifles and shotguns used in the United States.
Comparison with Ferrocerium
Use of flint and steel should not be confused with use of ferrocerium (aka "hot spark", "metal match", or "fire steel"). This man-made material, when scraped with any hard, sharp edge, produces sparks that are much hotter than obtained with natural flint and steel, allowing use of a wider range of tinders. Because it can produce sparks when wet and can start hundreds or thousands of fires when used correctly, ferrocerium is a common item included in survival kits. Called "flint", ferrocerium is also used in many cigarette lighters.
As a building material
Flint, knapped or unknapped, has been used since antiquity (for example at the Late Roman fort of Burgh Castle in Norfolk) up to the present day as a material for building stone walls, using lime mortar, and often combined with other available stone or brick rubble. It was most common in parts of southern England, where no good building stone was available locally, and brick-making not widespread until the later Middle Ages. It is especially associated with East Anglia, but also used in chalky areas stretching through Sussex, Surrey and Kent to Somerset. Flint was used in the construction of many churches, houses, and other buildings, for example the large stronghold of Framlingham Castle. Many different decorative effects have been achieved by using different types of knapping or arrangement and combinations with stone (flushwork), especially in the 15th and early 16th centuries.
Flint pebbles are used as the media in ball mills to grind glazes and other raw materials for the ceramics industry. The pebbles are hand-selected for colour, with those showing a reddish tint, indicating the presence of iron, being discarded. The remaining blue-grey stones have a low content of chromophoric oxides and so should impart lesser amounts of colouring contaminants.
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.