Flint

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This article is about the sedimentary rock. For other uses, see Flint (disambiguation).

A flint nodule from the Onondaga limestone layer, Buffalo, New York. (3.8 cm wide)

Flint (or flintstone, also known as silex) is a hard, sedimentary cryptocrystalline silicate form of the mineral quartz, categorized as a variety of chalcedony. Flint is usually dark-grey, blue, black, or deep brown in colour, and often has a glassy appearance. It occurs chiefly as nodules and masses in sedimentary rocks, such as chalks and limestones.

The exact mode of formation of flint is not yet clear or agreed but it is thought that it occurs as a result of chemical changes in compressed sedimentary rock formations, during the process of diagenesis. One theory is that a gelatinous material fills cavities in the sediment, such as holes bored by crustaceans or molluscs and that this becomes silicified. This could certainly explain the complex shapes of flint that are found.

[edit] Uses

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, 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).

Pebble beach made up of flint nodules eroded out of the nearby chalk cliffs, Cape Arkona, Rügen

Flint was one of the most commonly used materials for the manufacture of stone tools during the Stone Age, as it splits into thin, sharp splinters called flakes or blades (depending in the shape) when struck by another hard object (such as a hammerstone made of another material). This process is referred to as knapping.
When struck against steel, flint will produce sparks, which when directed onto tinder can be used to start a fire. This occurs when the hard flint knocks off a particle of the steel, which is heated by the impact, and then burns with oxygen from the atmosphere. This method is popular in woodcraft and among campers who want to have an 'authentic' experience. Striking a lump of flint against a piece of steel to make fire is not particularly easy or convenient (although it is much easier than other primitive fire-making methods such as using a bow and drill). Because of this, a similar technology has been miniaturized and integrated into lighters, which are easy to use without skill or practice.
Starting a fire with flint is a staple of scouting lore.
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.
Flint was used extensively from the 13th century until the present day as a material for building stone walls, especially in parts of England. In chalky coastal areas of England flint has also been used as a building and walling material, predating the common use of bricks but laid in a similar manner, using lime mortar. For instance, flint was used in the construction of many churches and other buildings in East Anglia.

Detail of flint used in a building in Wiltshire, England.

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 England flint pebbles were traditionally an important raw material for clay based ceramic bodies; after high temperature treatment, to remove organic impurites and induce certain physical reactions, calcined flint performed a similar role to quartz sand used in other countries, i.e. after milling to fine particle size was the filler component in pottery bodies. Because of this historical use American potters now, erroneously, refer to all siliceous fillers as flint.