Gold

• 1. Introduction

  ›

  Gold carries the chemical symbol Au, from the Latin aurum, and the atomic number 79. It is one of the least reactive elements known, sitting second from the bottom of the reactivity series, with only platinum ranked as less reactive. A bright metallic yellow when pure, it is dense, soft, and so workable that humans have shaped it for the whole of recorded history. Yet much about it resists easy explanation. Why is a metal that refuses to corrode the very first metal that people seem to have used. How did so many fortunes and frauds gather around a substance that, by mass, exists on Earth in astonishingly small amounts. Where did it come from in the first place, and why does it gleam yellow when nearly every other metal is gray or silvery white. These are the questions ahead, from the deaths of stars to the lavatory seats of an imaginary island.

• 2. Born In Catastrophe

  ›

  Gold in the universe was already present in the dust from which the Solar System formed. Scientists have traced it to three cosmic sources: supernova nucleosynthesis, collisions of neutron stars, and the giant flares of magnetars. All three rely on the r-process, rapid neutron capture, which builds elements heavier than iron.

  In August 2017, electromagnetic observatories caught the spectroscopic signatures of heavy elements, gold among them, during the GW170817 neutron star merger event. That single event generated between 3 and 13 Earth masses of gold. It confirmed, after years of only indirect detection, that such mergers forge the metal.

  Neutron star mergers occur only about once every 100,000 years, which left a puzzle. They happen too late and too rarely to explain the gold found in stars that formed early in cosmic history. In 2025, researchers resolved this paradox by confirming that magnetar flares also produce gold. A single such flare can create more heavy elements than the mass of Mars. Scientists estimate these flares may contribute roughly 1-10% of all elements heavier than iron in our galaxy.

  Getting gold onto a reachable part of the Earth was a second act of violence. Because the early Earth was molten, almost all its gold sank into the planetary core. One model holds that the gold now in the crust and mantle was delivered later by asteroid impacts during the Late Heavy Bombardment, about 4 billion years ago. The richest gold deposits on Earth, in the Witwatersrand basin of South Africa, sit near where an asteroid struck.

• 3. The Witwatersrand Story

  ›

  The asteroid that formed the Vredefort impact structure 2.020 billion years ago is often credited with seeding the Witwatersrand basin. The popular version of that story is wrong. The gold-bearing rocks there were laid down between 700 and 950 million years before the impact, and were already buried under Ventersdorp lavas and the Transvaal Supergroup when the meteor arrived.

  What the impact actually did was bend the basin. It tilted the gold-bearing rocks so they reached the present erosion surface in Johannesburg, just inside the rim of the original 300 km diameter crater. The deposit was discovered in 1886, launching the Witwatersrand Gold Rush. Roughly 22% of all the gold known to exist on Earth has come from these rocks.

  The gold itself lies only in six areas, where ancient rivers from the north and north-west built pebbly braided river deltas before draining into the so-called Witwatersrand sea. The basin is a layer of archean rock 5 to 7 km thick, dipping so steeply that some mining reaches depths near 4000 m. The Savuka and TauTona mines, south-west of Johannesburg, are the deepest mines on Earth. The city of Johannesburg itself was founded as a result of this rush.

• 4. Why It Is Yellow

  ›

  Most metals are gray or silvery white, but gold is slightly reddish-yellow. That single fact is a well-known example of relativistic quantum chemistry. When relativity is included in theoretical calculations, the 5d-6s band gap shrinks greatly, and this is thought to account for the color. A similar effect gives metallic caesium its golden hue.

  Gold is the most malleable metal there is. It can be drawn into a wire a single atom wide and stretched far before breaking. A single gram can be beaten into a sheet of one square metre, and an avoirdupois ounce into 300 square feet. Beaten thin enough, gold leaf turns semi-transparent, and light passing through it appears greenish-blue, because gold reflects yellow and red so strongly.

  That same thin gold reflects infrared light, which is why it lines the visors of heat-resistant suits and the sun visors of spacesuits. Mixing in other metals changes the color entirely. Copper makes eighteen-karat rose gold. Palladium or nickel makes white gold. Iron yields blue gold, aluminium yields purple gold, and silver alone produces a greenish-yellow known as green gold. Even colloidal gold shifts shade: small particles look red, larger ones blue.

• 5. The Noblest Metal

  ›

  Gold is the most noble of the noble metals, yet it still forms many compounds, with oxidation states running from -1 all the way to +5. The two that dominate its chemistry are Au(I), the aurous ion, and Au(III), the auric, whose complexes are typically square planar.

  Gold does not react with oxygen at any temperature and resists ozone up to 100 degrees Celsius. The halogens are another matter. Fluorine attacks it at dull-red heat to form gold(III) fluoride. Chlorine reacts at 180 degrees Celsius and bromine at 140 degrees Celsius, while iodine reacts very slowly. Unlike sulfur, phosphorus reacts directly at high temperatures to make gold phosphide.

  The acids mostly leave gold alone. It ignores hydrofluoric, hydrochloric, sulfuric, and nitric acid, but it dissolves in aqua regia, a 1:3 mixture of nitric and hydrochloric acid. Nitric acid alone, which eats silver and base metals, cannot touch it. That difference is the origin of the term acid test, long used to confirm the presence of gold.

  Its rarer states are stranger still. In aurides, gold takes the -1 oxidation state, helped by the highest electron affinity of any metal at 222.8 kJ/mol. Caesium auride crystallizes in the caesium chloride motif. There is a gold(II) complex, the tetraxenonogold(II) cation, that uses xenon as a ligand. And gold pentafluoride, with its related heptafluoride, marks gold(V), the highest verified oxidation state.

• 6. Fortunes And Frauds

  ›

  The earliest recorded metal used by humans appears to be gold. Small amounts of natural gold have turned up in Spanish caves from the late Paleolithic period, around 40,000 BC. The oldest gold artifacts in the world come from Bulgaria and date to the 5th millennium BC, including finds at the Varna Necropolis near Lake Varna.

  Mansa Musa, ruler of the Mali Empire from 1312 to 1337, made the metal infamous on his hajj. Passing through Cairo in July 1324, he gave away so much gold that he depressed its price in Egypt for over a decade, causing high inflation. Centuries later, the lure ran the other way across the Atlantic. The Aztecs called gold teocuitlatl, literally god excrement, and after Moctezuma II was killed, most of it was shipped to Spain.

  Gold also attracted spectacular liars. Prescott Jernegan ran a gold-from-seawater swindle in the United States in the 1890s, and an English fraudster repeated the trick in the early 1900s. Even serious science failed here. Fritz Haber studied extracting gold from sea water to help pay Germany's reparations after World War I. After analyzing 4,000 samples that averaged just 0.004 parts per billion, he ended the project.

  The deepest deception ran through alchemy. Producing gold from a baser substance like lead, supposedly by way of a mythical philosopher's stone, drove centuries of inquiry. Transmutation only became real in the 20th century. Japanese physicist Hantaro Nagaoka synthesized gold from mercury in 1924, and in 1980 Glenn Seaborg transmuted several thousand atoms of bismuth into gold at the Lawrence Berkeley Laboratory.

• 7. Money And Its Ghosts

  ›

  The first known coins containing gold were struck in Lydia, in Asia Minor, around 600 BC. The talent coin of gold used around the time of Homer weighed between 8.42 and 8.75 grams. After an earlier preference for silver, European economies returned to minting gold during the thirteenth and fourteenth centuries.

  The gold standard then governed money for generations before unraveling. Most gold coins stopped circulating as currency in the 1930s. After World War II, the Bretton Woods system tied nominally convertible currencies to fixed exchange rates. That ended in 1971, when the United States refused to redeem its dollars in gold, an event known as the Nixon shock. Switzerland was the last country to tie its currency to gold, ending the link by a referendum in 1999.

  Purity has its own language. The gold proportion of an alloy is measured in karats, with pure gold designated 24 karat. English coins meant for circulation from 1526 into the 1930s used a 22k alloy called crown gold for hardness. Modern bullion still honors tradition. The South African Krugerrand, first released in 1967, is 22k, while the special-issue Canadian Gold Maple Leaf reaches 99.999% purity.

  Gold's prestige has always invited ridicule. In his treatise Utopia, Thomas More imagined an island where gold is so abundant it is used for slaves' chains, tableware, and lavatory seats. When foreign ambassadors arrive draped in gold jewels, the Utopians mistake them for menial servants. Today the metal trades under the ISO 4217 currency code XAU.

• 8. The Working Metal

  ›

  Only 10% of the world's new gold goes to industry, yet its chief industrial use is unglamorous: corrosion-free electrical connectors. Gold resists oxidation, conducts well, and does not corrode, so it coats the contacts inside computers and other devices. The World Gold Council notes that a typical cell phone holds about 50 mg of gold, worth around three dollars. Multiply that by nearly a billion phones a year and the figure reaches into the billions of dollars from this application alone.

  Gold has long doubled as medicine. Two compounds, sodium aurothiomalate and auranofin, are used today to treat arthritis and similar conditions for their anti-inflammatory properties. In the 19th century gold had a reputation as a treatment for nervous disorders, applied to depression, epilepsy, migraine, and notably alcoholism. The isotope gold-198, with a half-life of 2.7 days, is now used in nuclear medicine and some cancer treatments.

  In the kitchen, gold carries the E number 175. Being inert, it has no taste and no nutrition, and leaves the body unaltered. Decorative gold flake dates back to medieval European nobility. Danziger Goldwasser, a herbal liqueur made in what is today Gdansk, Poland, and in Schwabach, Germany, still contains flakes of gold leaf.

  Winning that gold from the ground exacts a heavy price. Low-grade ore is ground and mixed with sodium cyanide to dissolve the gold, and cyanide spills from mines have killed aquatic life along long stretches of rivers. Producing a single troy ounce can dump up to thirty tons of waste ore. As of 2020, mining a kilogram of gold produces 16 tonnes of carbon dioxide, while recycling the same kilogram produces only 53 kilograms. In one recent symbol of changing meaning, the autism rights movement adopted gold as a sign of pride, since its symbol Au resembles the start of the word autism.