Gold

Why gold is gold

Colour, softness and an almost total refusal to react

Nearly every metal is silver-grey. Gold is yellow because its atom is heavy enough for relativity to matter: the 79 protons pull the innermost electrons to a fair fraction of the speed of light, the outer shells shrink and shift in response, and the energy needed to lift an electron from the filled 5d shell to the 6s level falls into the blue end of visible light. Gold absorbs blue and reflects the rest — which our eyes read as yellow. Silver, one row up in the same group, makes the same transition in the ultraviolet and stays colourless.

The same lone 6s electron leaves the atoms bound to one another loosely. Gold is the most malleable and ductile of all metals: a single gram can be beaten into a leaf about a square metre across, so thin that light passes through it green, or drawn into more than a kilometre of fine wire. Pure gold scratches with a fingernail, which is why jewellers alloy it with copper and silver and count purity in carats, 24 being pure.

And it does not react. Oxygen leaves it alone, water leaves it alone, and so do hydrochloric, sulphuric and nitric acid on their own. Only a mixture of nitric and hydrochloric — aqua regia, “royal water”, named for the metal it could dissolve — or a cyanide solution in air will take it into solution. A coin buried for two thousand years comes out of the ground as bright as it went in. That permanence, more than anything, is what people have paid for.

Where it comes from

Made in dying stars, hidden in quartz, found in rivers

Stars make elements up to iron by fusion; anything heavier has to be built by cramming neutrons into nuclei faster than they can decay. That “rapid process” needs an extraordinary flood of neutrons, and for decades supernovae were the assumed source. In August 2017 the gravitational-wave observatories caught two neutron stars spiralling into each other, and telescopes watching the aftermath saw the afterglow of freshly made heavy elements — the first direct sight of a gold factory. Neutron-star mergers now look like the main source, with some rare kinds of supernova contributing the rest. Whatever the recipe, all of Earth’s gold was made before the Sun existed.

On Earth it is scarce: about four parts per billion of the crust. Most of it sits in veins of quartz, deposited from hot, mineral-laden water that once rose through cracks in the rock. Where such veins weather, the gold — heavy, and indifferent to the chemistry that rots everything around it — settles in stream beds as flakes and nuggets. These placer deposits, worked with pan and sluice, are what set off the gold rushes of California in 1849, Victoria in 1851 and the Klondike in 1896. The single richest source ever found is the Witwatersrand basin of South Africa, an ancient buried river system that has yielded more gold than any other place on the planet.

Money and ornament

Gold turns up native — as metal, not ore — so it needed no smelting to discover. The oldest known worked gold, some three thousand beads and ornaments from the Varna cemetery on the Bulgarian coast, is about 6,500 years old. Egypt drew it from Nubia and buried its kings in it; Tutankhamun’s inner coffin alone weighs about 110 kg. The Lydians of western Anatolia struck the first gold and silver coins in the sixth century BC, and from then on gold coin was the money of empires: the Persian daric, the Roman aureus, the Byzantine solidus, the Florentine florin and the Venetian ducat.

In the nineteenth century the great trading nations tied their paper currencies to gold at fixed rates — the gold standard — and the discipline it imposed lasted, with wartime interruptions, until the Depression broke it. After 1944 only the dollar was convertible, at $35 an ounce; in August 1971 the United States stopped honouring even that, and for the first time in history no major currency was backed by anything but trust. Gold’s price was set free, and it has been an investment rather than money ever since.

Gold today

Mines bring up roughly three and a half thousand tonnes a year, China, Russia and Australia among the largest producers, and recycling adds well over a thousand more. Set against everything ever mined — some 210,000 tonnes, a cube about 22 m on a side — that is a trickle: gold is not used up, only moved. Jewellery takes about half of each year’s supply, India and China buying most of it; bars, coins and funds take much of the rest; and central banks, which hold roughly a fifth of all the gold in existence, have been net buyers since 2010.

The smallest share is the one modern life could least do without. Because gold conducts well and never corrodes, a few tens of milligrams sit in almost every phone and computer, on connector pins and bonding wires that must still work after years in the open. Thin gold films line the visors of astronauts and the mirror of the James Webb Space Telescope, reflecting almost all the infrared that falls on them without ever tarnishing. Gold nanoparticles carry drugs and mark tests, and salts of gold treated rheumatoid arthritis for much of the twentieth century. The metal chosen five millennia ago for its shine turns out to have been chosen for the right reasons.