by Chris Woodford. Last updated: November 1, 2016.
There aren't many things that everyone in the world can agree on—but there's one thing virtually everyone would admit: gold is valuable. People have been making precious articles from gold for over 5000 years and even now gold bars remain the bedrock of the world's financial system. You only have to look at a piece of gold or feel how heavy it is to understand its enduring appeal. Even the name "gold" conveys something of the value we've come to attach to this precious metal: it comes from an Indo-European word ghel meaning to "gleam" or "shine." Where does this amazing substance come from and what makes it so special?
Photo: Gold in space: Ed White became the first astronaut to step into space and simply "let go" in 1965. He's attached to his spacecraft by a gold-wrapped umbilical line and his face is protected from the Sun's harmful rays by a gold-coated visor. Photo courtesy of NASA on the Commons.
What is gold?
Photo: A sample of pure gold from Francomb Hill, Summit County, Colorado, United States. Photo originally by US Bureau of Mines courtesy of US Geological Survey.
Gold is a soft yellow metal that's amazingly easy to work. It's far more malleable (easy to shape) and ductile (easy to draw into wires) than any other metal. You can beat a piece of gold into a "leaf" so thin that you'd need 100,000 leaves like that to make a pile 1 cm (0.4 inch) high. Gold's amazing softness also means you can draw a single ounce into a wire 64 km (40 miles) long! Despite being soft, it's very dense—even the small amount of gold in a wedding ring feels surprisingly heavy.
The gold we use from day to day often needs to be much harder than this to survive rough treatment. Typically, we mix gold with another metal to make a substance called a gold alloy (an alloy is simply a mixture of a metal with one or more other metals and perhaps some other substances added too). We describe how much gold an alloy contains using a measurement called the carat (sometimes written karat). One carat is equal to 1/24 part: one carat gold is 1/24 gold and 23/24 other substances, 18 carat gold is ¾ (18/24) gold, and 24 carat gold is pure gold. The purity of gold can also be described with a measurement called the fineness. Pure gold is said to be "1000 fine," while something containing only ¾ gold (18 carat) would be described as "750 fine." The gold used in coins is usually 900 fine (90 percent gold mixed with 10 percent silver). How do you know how pure your gold is? There's usually a mark stamped into it (known as a hallmark) to indicate its purity in carats.
One reason gold is so valuable is that it's chemically very unreactive. Air, heat, water, and most other substances have little or no effect on it. That's why chemists describe gold with words like "stable," "unreactive," "inert," and "noble"—all of which mean it's reluctant to take part in chemical reactions. Gold does react with acids and bases, however, and with cyanides and halogens.
Where does gold come from?
As prospectors throughout history will tell you, gold isn't that easy to discover: although you can find it in many different countries in all kinds of rocks and soils, it's only the 75th most common chemical element in Earth's crust. Given that there are only 90 or so natural elements, that's another way of saying gold is pretty rare. Since gold is so unreactive, it's possible (but unusual) to find pure gold in mineral deposits; typically most gold is combined with other elements in alloys such as electum (a mixture of gold and silver) and gold amalgam (gold and mercury) or combined with other metals including quartz, lead, and zinc. Most gold comes from three different kinds of mineral deposits: vein (lode) deposits (where the gold occurs in cracks in rock, a bit like the veins in blue cheese); placer deposits (where vein deposits are worn and washed away by rainwater and settle into the beds of streams); and as part of copper-containing minerals known as porphyry copper deposits. Surprisingly, gold is also present in seawater in quantities of a few parts per million—too little to make it worth extracting.
Source: Which countries produce the world's gold? Chart shows estimated figures for world mine production, 2015. Source: U.S. Geological Survey, Mineral Commodity Summaries, January 2016.
How do we get gold out of the ground?
If you've ever watched movies about the Californian gold rush, you'll have seen grizzly looking prospectors panning for gold from placer deposits in streams. Panning means using a shallow pan to wash away sand and gravel until heavier nuggets of gold remain. Modern gold mining is far more sophisticated and happens on a much bigger scale.
Placer deposits are mined using a scaled-up version of panning. Huge mechanical excavators and dredges pull out vast amounts of material from the bed of a river or creek. The material is then washed and shaken to separate the gold out from the useless waste (known as gangue). Gold mined from underground deposits is mixed in with gravel and rock and has to be extracted by a mixture of physical and chemical processes. The exact process varies from place to place and depends on what other elements the gold is combined or mixed in with. Typically, gold-containing ore might be crushed, dissolved in mercury or cyanide, smelted, roasted, or subject to various kinds of filtering (including floatation and gravity separation—where heavy gold is allowed to settle to the bottom of a container) before the gold is pure enough to be melted and shaped into bars called ingots. Gold can be purified by chemical means or by electrolysis (passing an electric current through a gold-containing solution).
Photo: A gold olive branch (a traditional symbol of peace) left on the Moon by Neil Armstrong and the Apollo 11 space mission. Photo courtesy of NASA on the Commons.