Flint is a ubiquitous sedimentary cryptocrystalline form of quartz. It is found across the world, in regions that have sedimentary rocks, such as chalks and limestones.
It mainly appears as 'nodules' which are usually grey, black, green, white or brown in colour, and often have a glassy or waxy appearance.
It was formed in the Jurassic and Cretaceous periods, but the geological processes which formed the flint are unknown.
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.