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.http://www.bbm.me.uk/portsdown/PH_320_Flint.htm The Flints from Portsdown Hill 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.

Tools

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).

Fire

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. The components are also part of many survival kits, such as those issued by many military units, because they have the ability to work while wet, have a very long shelf-life, and will last a very long time in an extended survival situation (unlike matches or a lighter which not only need to be specially design to be able to light when wet, but which would be exhausted sooner than a flint-and-steel fire starter in a longer-duration survival scenario). 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, and requires little practice to employ). Matches and lighters are a much quicker and more convenient way of starting a fire, but have the disadvantages of being unable to function when wet (except for special designs) and of being more limited (than flint and steel) in the number of times that they can be used before exhausting their fuel supply.