What is flint?

Flint (chert) is a form of quartz, or silicon dioxide, also called silica. It occurs in layers and irregular nodules in the chalk and some other limestones. It is widely distributed around the world and was a primary material for stone age tools and weapons. Chemically, flint is complex. It is a fine mosaic of colloidal silica (opal) and crypto-crystalline silica (chalcedony).

Chalk beds were formed on the floor of ancient seas in the Cretaceous period between 145 million and 65 million years ago. Chalk is made up of individual crystals of calcium carbonate, from the bodies of microscopically small sea creature called Coccolithophores. These formed a whitish mud which later solidified into limestone rock. Flint occurs both as layers and as irregular nodules in these chalk beds. There are extensive chalk deposits in the United Kingdom, France, Denmark, Poland, Russia, Ukraine, United States, Canada, and elsewhere. All contain flint.
		A coccolithophore. (The white bar just visible at the bottom left is 1 millionth of a metre long.) Chalk is formed of plates from its body.

		Details of how the flint formed are still uncertain. The silica material is probably derived from the siliceous spicules of sponges and microscopic siliceous plankton, which are slightly soluble in water, so some of it dissolved in the ocean and was later precipitated out onto the chalk sea bed. Often it collected around some solid object such as a sponge or coral, and these are sometimes visible inside the flint when a nodule is broken open. It’s thought that initially, the silica must have gone through a gel-like phase before hardening into flint, because it is found completely filling shells such as those of sea urchins. Where flint nodules from the chalk have accumulated in superficial deposits, such as clay-with-flints, it is common to find internal casts in flint of the shells that were once filled. Flint nodules also formed in the cavities left in the sea floor by burrowing marine animals, hence their shapes.
A flint nodule weathering out of chalk beds near Dover.

Flint nodules in the chalk changed over time as their outside acquired a whitish-coloured cortex or rind, possibly through ground water percolating through the chalk. It’s thought the cortex formed because the flint surface is slightly more soluble in some microscopically small areas than in adjacent areas, so there is differential dissolving at the surface which results in a microscopic sponge-like structure. The cortex can be 5mm or more in thickness. When people started making tools from flint, they often removed the external cortex to get at the fresh flint inside, although in some cases they left a small amount of cortex to make some tools, such as scrapers and knives, easier to handle.
		Internal flint cast of a sea urchin.

Various minerals can be included within the microscopically small cryptocrystalline structure of flint. Different mineral inclusions give rise to different colours. Freshly broken Flint is commonly black, grey, green, white or brown. The following semi-precious stones are all forms of quartz similar in structure to flint: Agate, Carnelian, Chalcedony, Jasper, Obsidian, Onyx and Opal. The microscopic structure of flint is capable of holding water molecules – as much as 30 per cent of the mass can be water, though it is typically 10 per cent. Some or all of this water may evaporate and leave the surface causing changes in colour and making the flint more brittle. Opal, for example, has a water content as high as 20 per cent, and often loses this water, and its rainbow colouration over time.

Left: different coloured flint implements from China and (right) Opal in its natural state.

On the international hardness scale, flint ranks 7 out of 10, where diamond is 10, so it is harder than most materials commonly encountered in the natural environment. It also has the property of taking an edge thinner than a steel blade (only a few molecules thick) so it is literally sharper than a razor. Flint is still in use today as surgical tool because incisions made with a flint blade heal more quickly and are more sterile.

Flint does not have a regular crystal structure, like diamond, that would enable it to be cut into regular shapes like gemstones. Because of its cryptocrystalline structure, it shatters in a conchoidal or cone-shaped fracture, like glass. This property has a number of implications for someone wishing to make flint tools. When you strike a flint core with a hammerstone, you will always create a fracture cone spreading out from the point of impact. If you hit near the edge of the core, and at the right angle, the energy from your blow will strike off a flake. If you hit too far from the edge, or at the wrong angle, the energy from your strike will simply create a fracture cone inside the core. One or two mis-hits like that and your core becomes an unusable mass of intersecting fracture cones. Flint is thus very unforgiving to the inexperienced user. A second implication is that one cannot cut or carve flint at will as one can with most softer rocks – it can only be struck into flakes or blades in a specific chain of operations that makes use of its natural method of fracturing.

When a tool was discarded after use in prehistoric times, a freshly knapped tool in many cases began to weather, acquiring a patina over its external surface. This patination can be affected by up to four distinct and separate processes. First, there is the differential solution of microscopic areas referred to earlier. The acid in rainwater will dissolve the more soluble opaline silica from the surafce, leaving a porous mesh of minute crystals that scatter light, creating a new cortex that appears first blueish than whiteish in colour. This grows in thickness over time and can be a millimetre or more in some cases. Usually, however, patination this thick is seen only in implements from the Palaeolithic, tens of thousands of years old. in most flint implements from the Mesolithic and Neolithic periods, the patina (if any) is superficial only. The British Museum's 1950 handbook on Flint Implements refers to an experiment in which flint was continuously submerged in rainwater charged with the products of decaying vegetation in the presence of chalk. Twenty-two months were needed to produce a patina 0.01mm thick.

Stages in colour-change of patination of flints as a cortex develops: Black when freshly broken, later acquiring a thicker and lighter patina through weathering. The flint flake shown far right is from chalky soil.

Where flints are buried in contact with the chalk, or with chalky soil, they tend to develop a white patina as in the flake shown above right. Third, where the implement lodges in ground containing iron minerals, the whitish patina absorbs the reddish or brownish ferrous material. And fourth, there can be an additional complex process in which molecules of quartz are dissolved from the surface and redeposited again forming a glossy coating of opal over the existing patina. This glossy coating or sheen is different from the gloss caused by wind abrasion in desert environments (desert gloss) and gloss caused by use wear (sickle gloss). There is a further patination feature associated with some flint tools and that is the occurrence of brownish spots mainly on the ridges between flake scars. This is known as ‘iron mould’, because its appearance resembles the spreading of a microorganism like mould.

It is common to find flints, both in worked form and as natural nodules, that have been calcined or heated by fire. Heating changes the appearance of flint to a grey, sometimes glossy surface, often cracked like the glaze on old pottery. In many cases, these calcined flints are associated with camp sites, and hence are believed to be hearth stones or ‘pot boilers’ – that is stones that have been heated in the fire and then placed in a container of water to boil it. Some experimental archaeologists think that people may have heated flint to some extent to make it easier to work, though this question is still debated.

Museum of the stone Age