FLINT (a word common in Teutonic and Scandinavian lan guages, possibly cognate with the Gr. irXivOos, a tile), in petrology a black, grey or brown form of hydrated silica which has an almost vitreous lustre, and when pure appears structureless to the unaided eye. When unaltered by weathering, it is, in the mass, dark and opaque, but in thin plates or on the edges of splin ters, it is pale yellowish or pale grey. Its hardness is greater than that of steel, so that a knife-blade leaves a grey metallic streak across its surface. It is brittle and if finely broken yields angular chips which are used as an abrasive. The fracture is shell-like, and when resulting from a blow, perfectly conchoidal, so that a "cone" or "bulb" of percussion is formed. Under thermal action (heat or frost) shell-like flakes are removed, but the bulb of per cussion is absent. Although the flint appears to be homogeneous, experienced flint "knappers," such as those engaged in gun-flint manufacture at Brandon, Suffolk, recognize a "grain" which per mits easy flaking in certain directions. For the removal of smaller flakes, pressure from another stone is sufficient.
When flint is examined microscopically, it is usually found to be very finely crystalline and to consist of quartz and chalcedonic silica; some varieties, however, appear to be amorphous, re sembling opal. Sponge-spicules and fragments of other organisms, wholly or partially silicified, with retention of their structures, may be present.
In Britain the term flint is restricted to concretionary nodules, veins and tabular masses found in the Chalk (see CRETACEOUS), and as pebbles in the gravels, etc., of Tertiary and Quaternary age derived from its erosion. The coarser and less homogeneous similar substance occurring in the older rocks (Carboniferous Limestone, Devonian, Ordovician and pre-Cambrian) is known as Chert. Some cherts contain tests of Radiolaria and thus resemble superficially the radiolarian oozes forming in the oceans at the present day.
The principal use to which flint has been put is the fabrication of weapons in Palaeolithic and Neolithic time (for which see FLINTS). The making of gun-flints may be regarded as a survival of this industry. The rounded beach-pebbles of flint from the South Coast of England and the north of France are used for ball-mills in which pottery-materials, sand, etc., are finely ground. Chert-slabs from the Carboniferous and Cretaceous rocks are used similarly in pan-mills. Ground flint is used as a source of silica in glazes and pottery, and as an abrasive. Its use in glass (e.g., for flint-glass) has long been abandoned. The use of flint gravels for road-mending, is now confined to by-roads.
(P. G. H. B.)