Flint, a mineral which may be regarded as a variety of quartz, allied to chalcedony, consisting almost entirely of silica, with a very little lime, oxide of iron, water, carbon, and sometimes even traces of organic matter. It has a flat shell-like fracture, is translucent or semi-transparent, and varies in colour from a very dark brown, or almost black, to light brown, red, yellow, and grayish-white, and is sometimes veined, clouded, marbled, or spotted. Dark-coloured flints are most common in the chalk, in which principally flint occurs imbedded, forming tabular sheets and nodules of various sizes, sometimes large nodular masses, of irregular and often grotesque shape; but gravel formed of light-coloured flints is very common, and it is disputed whether or not a change of colour has taken place by exposure to atmospheric and other chemical agencies. Flint is sometimes found in beds or veins. It is very abundant wherever the chalk formation extends, in England and other countries; rolled flint nodules are also often found in conglomerate rocks, and in alluvial soils—vast alluvial tracts being sometimes full of them. Flint geodes often contain crystals of quartz. Flint nodules are usually moist in the interior if broken when newly taken from their beds.
Flint is sometimes harder than quartz, sufficiently so to scratch it. The readiness with which it strikes fire with steel is well known, and it would seem that the sparks are not all merely incandescent particles, heated by the friction, but that in some of them a chemical combination of silica and iron takes place, causing great increase of heat. The use of the flint and steel for igniting tinder, once so common, has been almost superseded by that of lucifer matches, and gun-flints have given place to percussion-caps. The most ancient use of flint was probably for sharp weapons and cutting instruments; and flint knives, axes, arrow-heads, &c. are among the most interesting relics of rude antiquity. In East Anglian churches squared flints have been used for centuries to ornament the porches, towers, buttresses, &c.; but at present the principal use of flint is in the manufacture of fine earthenware, into the composition of which it enters, being for this purpose first calcined, then thrown into cold water, and afterwards powdered.
The origin of flint is a subject of considerable difficulty. Siliceous deposits are sometimes the result of a purely chemical operation, as in the case of the siliceous sinter formed round the geysers of Iceland, from the evaporation of water largely charged with silica. But at the bottom of the sea, as no evaporation could take place, some other agent than springs of water saturated with silex must have supplied the materials. It is a fact of considerable importance in this inquiry that almost all large masses of limestone contain siliceous concretions, or flints. Thus, chert is found in carboniferous and other limestones, and menilite in the tertiary limestones of the Paris basin. The conditions necessary for the deposition of calcareous strata seem to be those required for the formation of siliceous concretions. The materials of both exist in solution in sea-water, and, as it needed the foraminifer, the coral, the brachiopod, and the mollusc to fix the carbonate of lime which formed the chalk deposits, so the silex was secreted by innumerable diatoms and sponges, and their remains most probably supplied the material of the flint. The discovery by Dr Bowerbank and other microscopists of the spicules of sponges and the frustules of diatoms in almost every specimen of flint has clearly shown that flint to a large extent, if not entirely, owes its origin to these minute organisms. After the death of the organisms their silica appears to have been redissolved and redeposited, perhaps through the agency of decomposing animal matter, sometimes in the form of irregular concretions, and sometimes replacing the calcareous skeletons and exuviae of other organisms.