Long Read: Sand doesn’t just stick between our toesit also has a way of getting inside our heads. Rebecca Willis finds eternity, and more, in a grain of it

CLOSE YOUR EYES and picture this. You are walking in the sunshine under a blue sky. On one side of you is a green mass of palm trees, on the other the turquoise of the sea. And under your bare feet is sand, white sandpowdery and silky, soft yet firmwhich yields and then holds as you step on it. It sends a sensual thrill from the soles of the feet up into your brain.

Now change the picture. Make the sand beneath you coarser. Turn it to gravel. Make it sharp. It doesn’t work, does it? The sand is essential to the scene. And even if you de-saturate the colour, even if you have rocky cliffs instead of palm trees to your left and a steely sea to your right, the sand under your feet—which may now be greyish—still makes you want to take off your shoes and wriggle your toes into it.

To the travel industry, every beach is white. And it is no coincidence that white sand is, for most of the developed world, a long-haul flight away and associated with wealth, just as a tan was in the early years of the jet engine. The things we desire, or are encouraged to desire, often follow the money.

The place the beach occupies in the Western imagination today has changed dramatically in the 300 years since "Robinson Crusoe" was published. Then it was a hostile, dangerous frontier, next to the wild unknown of the sea. It reeked of shipwrecks, invasions and the treacherous business of fishing. But it is telling that even then Daniel Defoe transposed the tale of Alexander Selkirk, in part his inspiration for Crusoe, from a temperate, rocky island off the coast of Chile to a sunny Caribbean one with beaches. Sailors exploring the South Pacific were understandably seduced, after many hard months at sea, by the warm waters, fresh food and sexual freedom of the islanders, and their tales travelled back to Europe. But the reality was that, until man’s dominion over nature became more assureduntil sun cream and vaccinations against diseasesthe tropical seaside was an unfriendly place to find yourself, sometimes fatally so.

Pale sand may be most prized, but sand of any shade has a hotline to our senses; we want to touch it and mould it and play with it. That is why hotels in the Caribbean don’t replace their beaches with concrete, even though they may be in annual danger of being whisked away by a hurricane. Perhaps warm sand beneath our soles triggers atavistic memories of our ancestral home in Africa, or perhaps it is simply the opposite of our hard, urban streets. Either way, sand exerts its magnetism with extra force at this time of year, the holiday season for the northern hemisphere. But what exactly is this stuff that draws us irresistibly to the coast? How did it develop the power to create new migration patterns in homo sapiens? And why has it lodged itself so firmly in our collective psyche?

I GREW UP in a village called The Sands, near which some of the best building sand in England is gouged out of the earth by huge diggers (sand has been used in concrete since it was invented by the Romans 2,000 years ago). Attending church on Sundays as children, we used to singwithout any sense of ironya hymn with the chorus: "Oh, build on the rock and not upon the sand...". We were too young to realise that this was just a metaphor for religious purposes, exploiting the shifting nature of sand. And we didn’t know that in fact well-drained, well-compacted sand makes a good base for building on. The actual sandpits were strictly out of bounds to us and had an aura of strange magic: there were rumours of escaped pet terrapins which had grown to gargantuan size and lurked in their muddy waters, and of teenagers drowning in twining weeds that pulled them under the surface. These myths, perhaps started by parents, cast their own spell: we did not venture beyond the barbed wire.

Sand used for building, such as this, and the sand of beaches outside the tropics70% of the world’s sand, in factis made of quartz, also known as silica, produced by the grinding and scouring of millions of years of weathering and glaciation. The sand of tropical beaches is different. It is "biogenic", or produced by life processes, and consists largely of calcium carbonate: the ground-up remains of shells, coral and the skeletons of marine creatures (the parrotfish is known as the “sand maker” because it feeds on coral and excretes sand). That is the simple explanation for the difference in coloursand in the English Channel is never going to be travel-brochure white like sand in the tropics, even if the sun does shine.

The composition of sand varies greatly according to the rocks and conditions, but it is defined on the invariable Udden-Wentworth scale, which uses sieving to determine and average out grain size: from 0.0625mm to 2mm is sand, anything bigger is gravel, and anything smaller is silt. The smallest grains of sand are invisible to the naked eye, and a grain of sand starts its life the size of the crystal that made up its parent rock. Because this scale applies to all granular material, it means that salt and sugar are technically sand. Come to think of it, a beach made of sugar might be just the thing for modern man.

Of all granular materials, those we call sand are the most mobile in water. Mud sticks, stones are too heavy: sand is the traveller of the granular family, riding the winds and the waves. And like a traveller, it has a tale to tell. The type of mineral betrays its place of origin, a fact now used to help solve crimes. The shape speaks of its journeysdesert grains are rounder than sand eroded by water, which cushions it. And sand reveals age, too: exposure- or luminescence-dating measures the amount of radiation to which it has been subjected, and can determine the age of archeological finds way beyond the reach of carbon dating, as with the cave paintings of the Kimberleys in Australia, thought to beat up to 60,000 yearsthe oldest images we have of the human figure.