For countless generations, the mysterious pockmarks that scarred a tract of heath on the edge of Norfolk inspired curiosity and wonder. Who or what could have gouged out the 360 or so depressions that gave the area its decidedly lunar appearance?
The Saxons attributed them to Odin, the Norse god of the dead, and because Odin had the nickname Grim, the site became known as Grim's Graves. Not until 1870 did somebody dig deep into one of the hollows and discover it to be the top of a shaft 10 metres deep that was filled almost to the brim with chalk and soil.
Thorough excavation showed that, far from being the devil's work, the strange landscape was created by Neolithic flint miners. In recent years, painstaking analysis of the debris in and around the shafts has revealed the impressive scale of this ancient undertaking. We now know Grimes Graves to be the remains of a sophisticated industrial operation that produced and exported enough high-grade flint for the manufacture of some 5 million tools dating from around 3,000bc until well into the Bronze Age (approximately 2,750-700bc).
Clearing the decks
For 300 years or more, this seven-hectare clearing, in what is now a vast and lonely pine forest, was the centre of what must have been, in a very literal sense, the cutting-edge technology of its day. For without the black, glassy nodules so prized by the first miners, human society as we know it might never have got off the ground.
It was the unique ability of flint to produce a hard, razor-sharp edge that allowed the first farmers to clear land on which to grow their crops, and to perform the myriad tasks that were to set them apart from their hunting, gathering forebears. Axes, adzes, knives and scrapers were all crafted from this shiny mineral, indeed, it's possible to say that human civilisation was founded on a bed of flint.
Yet just as the lunar landscape of Grimes Graves remained indecipherable for centuries, so the origin of flint itself has long baffled geologists.
It occurs for the most part in shallow seams buried deep within chalk deposits. But how did it get there? And why does it so often take the form of gnarled, almost organic nodules?
These nodules can often be seen in Britain's rolling fields of chalk, lining the tops of garden walls or simply discarded on field margins. With their twisted, often perforated shapes, they resemble fossilised animals or plant structures.
Whatever appearances suggest, these are not fossils. Only recently has it been possible to piece together a probable sequence of events that led to the creation of flints - a sequence that begins with a form of microscopic sponge.
Known as "spicules", these creatures have internal skeletons composed of silica, and it is thought that these skeletons broke down and dissolved into a gelatinous, fluid mass that collected on the beds of oceans at a time when chalk deposits were being laid down, between 145 and 65 million years ago.
As it seeped down into the young chalk, the siliceous gel, composed of silicon dioxide, thickened and crystallised - a process that may have begun when it reached a particular level in the chalk. This would explain why shallow beds of black flints can often be seen in white chalk cliffs.
Crystallisation might also have been "seeded" by nuclei of marine detritus, which would explain why flints often contain fossils. Sometimes the gel would have concentrated and solidified inside fissures in the chalk, and even "grown" to fill the burrows of shrimps and other marine creatures.
In this sense, at least, generations of country people were right. The weird flint nodules whose organic forms were to be as suggestive to the lewd field-workers of Thomas Hardy's Tess of the d'Urbervilles as they were to the sculptor Henry Moore, did indeed "grow" in the chalk. And it was lucky for our early forebears that they did.
On the edge
For all its glassy appearance, flint is actually composed of microscopic crystals. Yet when struck hard, it fractures in a similar way to glass, releasing a wide, concave flake and leaving an edge that is as sharp as a modern scalpel. In fact, the sharpest scalpels in use today have a cutting edge of obsidian, which is a volcanic glass of identical chemical composition to flint.
With a succession of carefully aimed blows on a piece of high-quality flint such as that extracted from the Grimes Graves mines, an experienced "knapper" could produce tools to perform a variety of tasks, from felling trees to boring delicate holes in jewellery. Spearheads and arrowheads made from flints were also prized by hunters and warriors.
This was the same the world over, for wherever flint was to be found - in Africa, the Americas, Australia or the Far East - humans sought it out and learned how to fashion it into their most valuable possession: the blade.
Little wonder then that the demand for the mineral continued long after the arrival of bronze. There is even some evi-dence that flint was used during the Iron Age, as metal implements must have been luxury items in that period and available only to the rich.
The silica nodules would also retain their value even after their usefulness as blades diminished. The settled societies they had once made possible soon found another use for them, albeit a less glamourous one to begin with. Strip the marble facing from all but the grandest Roman building and you will expose an inner core of rubble - small stones of all sizes bound together in mortar. The Romans were masters of this early form of concrete, and in areas where flint was the most available stone "flint rubble", strengthened with regular courses of brick or tile, became the norm.
While anonymous lumps of silica were to provide the tough backbone of fortresses and town walls for centuries, it was during the Middle Ages in Europe, and especially in East Anglia, that stonemasons came to recognise its decorative potential.
Art of darkness
Once again, it was flint's glassy black interior that proved its unique selling point. From the 14th-century, split flints began to appear on the facing of walls, usually in combination with, and in the starkest contrast to, white limestone. Entire buildings were decked out in glinting chequer patterns, like gigantic chessboards set vertically against the sky.
In the very finest churches, particularly those financed by the burgeoning wool trade, flint and limestone were carved into elaborate tracery patterns known as "flushwork", which rivalled stained glass windows in their complexity and artistry. It was, once more, a good time to be a flint worker. While the fashion for black and white buildings was relatively short-lived, another of flint's striking qualities - "striking" in a literal sense - ensured that this mineral, together with those who extracted and worked it, continued to be in demand for centuries to come.
Strike a light
The word "flint" has become to be synonymous with hardness for good reason.
On a scale of one to 10, with diamond at the top and talc at the bottom, flint has a relative hardness of 6.5, which makes it roughly on par with the harder forms of steel. It follows that if a piece of hard steel is struck at an angle by a sharp flint edge, then a tiny sliver of the metal will be sliced off - a process which generates so much heat that the sliver of metal becomes white hot and molten as it flies through the air, and appears as a brilliant spark.
In an age when gunpowder was taking over from the blade as the decisive factor in warfare, this meant that after a gap of several thousand years flint was once again a vital ingredient in weapons technology.
In the earliest days of firearms, a smouldering taper was used to ignite the charge of powder. But in the turmoil of battle, keeping a taper alight often proved difficult and even dangerous. Clearly, some means of producing fire on demand was needed.
Flint and steel had long been used as a means of igniting tinder, and in 1515 a locksmith fitted a musket with an ingenious device which pressed a shard of flint against a milled steel wheel, thereby producing sparks that flew into the pan and ignited the powder.
Reinventing the wheel lock
The wheel lock, so called because its delicate moving parts resembled those of a lock, was replaced in the mid-16th century by a simpler and more robust mechanism called the flintlock, although the milled-wheel principle survives to the present day in some cigarette lighters.
Although a system of ignition based on percussion did away with the need for sparks in firearms early in the 19th century, flints for flintlock rifles continued to be made in Britain for export to South America until the middle of the 20th century. These used the finest quality flints available, which were knapped in the open air (the perils of silicosis were by now familiar) in a single knappers yard not far from their source. And where was this yard? In the town of Brandon, on the Suffolk-Norfolk border, just a few miles from Grimes Graves.
Caught knapping again
Despite having come full circle, the story of flint is far from finished.
In recent years, flint knapping has become a popular pastime worldwide, and in the US an enthusiastic knapping revival is underway, driven by a renewed interest in native American crafts. Those unlucky enough to live hundreds of miles from the nearest flint beds can buy high-quality "blanks" on the internet, and there is no shortage of books, videos and magazines to help the novice knapper to get started.
International conventions known as "knap-ins" offer the opportunity for enthusiasts to meet up for a knap and a natter, and Grimes Graves even has a resident flint-worker who demonstrates the craft several times a year.
These days, of course, knapping is strictly a leisure pursuit, although what some of the modern artifacts lack in antiquity they more than make up for in terms of beauty. It is a sobering thought that unless we tread carefully flint might once again come to represent the cutting edge of human technology.
It was Albert Einstein who put it most succinctly:"I know not with what weapons World War III will be fought, but World War IV will be fought with sticks and stones."