Gold an enduring icon of wealth, beauty and power. Since the beginning of recorded history, the rise of influential societies has coincided with their ownership of this prescious substance.

Its Latin name, aurum, means "glowing dawn." Though extremely rare, it is found on nearly every continent. It is prized above all other metals. It is gold.

With its distinctive combination of qualities, gold may well have been the first metal worked by humans. It was easily visible in stream- and riverbeds; it was easily shaped because it is soft; its alluring luster never grew dull. Over thousands of years, the pursuit of gold launched explorers, built empires and inspired artists. Gold itself became a symbol of wealth, beauty, purity, spirituality and the afterlife.

Today, gold is increasingly difficult to mine, but the demand for gold continues to grow. Gold's high status and value are unsurpassed around the world, its pivotal role in human history unending.

Gold crystals are usually worth much more than the value of the gold by weight. Their beauty is completely natural.

Gold is one of the few minerals that occur in a nearly pure, or native, state.

Only one out of a billion atoms of rock in Earth's crust are gold.

In nature, gold deposits occur as veins or as placers.

Most crystalline gold comes from hydrothermal fluid, extremely hot water rising from deep in the Earth. As the fluid moves through openings in Earth's rocky crust, tiny amounts of gold dissolve into it. Then, as the fluid flows through cooler rocks near the surface, the gold precipitates, or is drawn out of the fluid, and settles in cracks to form veins or lodes.

Over millions of years, gold flakes and grains worn away from veins are swept into bodies of water. The heavy gold settles in stream-, lake- and riverbeds, and on the sea floor, forming placer deposits.

Microscopic particles of gold can be extracted from rocks like quartz pebble conglomerates. Which comes from South Africa, for more than a century the world's most productive gold-mining country.

After gold-bearing rocks are pulverized, the gold is recovered using chemical processes.

Nearly 40 percent of all gold ever mined was recovered from South African rocks. Infinite Variety

Gold nuggets are solid lumps of gold. The term "nugget" was first used for gold in 1852 during Australia's gold rush.

Nuggets are often named for their appearance. The variety of names reflects the many possible shapes of native gold.

The rounded surfaces of many of these nuggets show that they were worn smooth in streams or rivers and collected from placer deposits.

Nuggets are rare, making up less than 2 percent of all native gold ever mined.

The Science of a Noble Metal

Gold, by any standard, is unusual. It resists chemical corrosion and tarnish-attack by acids and oxygen. It is highly reflective and an excellent conductor of electricity. Gold is dense but soft; it can be readily stretched, beaten and molded. It is prized for its rich color, luster and rarity; it is the only yellow-colored native metal. But what makes gold truly unique is that it combines all of these properties.

No Other Metal or Mineral Measures Up to Gold

Gold's beauty, value and its many other unique qualities make it the material of choice in many industries. Most gold—78 percent of the yearly gold supply—is made into jewelry. Other industries, mostly electronics, medical and dental, require about 12 percent. The remaining 10 percent of the yearly gold supply is used in financial transactions.

Gold is used as a contact metal in the electronics industry as it is a good conductor of both electricity and heat.

The word "gold" most likely has its origins in the Indo-European word ghel, meaning "yellow." The chemical symbol of gold, Au, is short for the Latin word for gold, aurum, meaning "glowing dawn."

Gold is dense, Gold is ductile: it can be drawn out into the thinnest wire.

Gold conducts heat and electricity. Copper and silver are the best conductors, but gold connections outlast both of them because they do not tarnish. It is not that the gold lasts longer, but that it remains conductive for a longer time.

Gold is ductile: it can be drawn out into the thinnest wire. One ounce of gold can be drawn into 80 kilometers (50 miles) of thin gold wire, five microns, or five millionths of a meter, thick. This sample is 0.20 millimeters (0.008 inches) in diameter.

Gold is highly reflective of heat and light. The visors of astronauts' space helmets receive a coating of gold so thin (0.00005 millimeters, or 0.000002 inches) that it is partially transparent. The astronauts can see through it, but even at that thinness the gold film reduces glare and heat from sunlight.

Gold is prized for its beauty. Jewelers and metalsmiths value it as a metal that can be embossed, hammered, cast, stretched or twisted.

Gold's Purity and Fineness

Karats are a measurement of gold's purity; a karat is 1/24 part, by weight, of the total amount.

Karats are a measurement of gold's purity; a karat is 1/24 part, by weight, of the total amount. Pure gold is described as 24 karats. Because gold is a soft metal, it is often alloyed, or combined, with other metals to make it harder. A metal alloy that is 75 percent gold is measured as 18 karats. Gold content can also be described by its fineness, or gold parts per thousand. Gold containing 25 percent of other metals is said to have a fineness of 750.

Gold Color

The color of gold changes when it is alloyed with other metals. Green gold and white gold—which looks like platinum—are used by jewelers. Yellow and pink gold are used for coinage and jewelry.

Microscopic crystals of gold take on the cubic shape dictated by the arrangement of its atoms. As one crystal builds upon another, the result is a mass big enough to be visible to the unaided eye, showing eight, twelve or even more faces, or sides. Individual gold cubes are never larger than two centimeters and are extremely rare. However, clusters of parallel growths of gold crystals are much more common, often taking on familiar forms known as wire, leaf, branch or tree. All of these forms use the same crystalline building blocks.

Heavier Than It Looks

Because of its density, a small amount of pure gold can be surprisingly heavy. A one-foot square cube is about the size of one ton of pure gold. A ton of iron would be a cube more than twice this size.

Colorado Bounty

Mine owner John F. Campion was a founder of the Colorado Museum of Natural History, now the Denver Museum of Nature and Science. His personal collection of minerals, donated to the museum in the early 1900s, includes examples of spectacular crystalline wire and leaf gold from the Farncomb Hill area of the Breckenridge district. The examples have been cleaned, probably with hydrochloric acid, to remove the surrounding minerals and reveal the natural structure of the gold crystals. These examples weigh as much as 628 grams (20 troy ounces).

Some wire and leaf gold formations come from pockets—openings as much as two or three feet in diameter—in the gold-bearing veins of the host rock. Pressure from hydrothermal—hot water—fluids forces open the crack and fracture in the rock. Then fluids deposit minerals where the gold crystals have room to grow.

Rare Combinations

Almost all gold mined on Earth is native gold; that is, the gold is in a pure or nearly pure state. Unlike copper, silver, iron and other metals, gold rarely combines with other nonmetallic elements to form complex minerals. It is this quality that also makes it resist corrosion.

However, a few minerals occur in nature that are chemical combinations of gold and nonmetallic elements such as sulfur, selenium and tellurium. These nonnative minerals are rare. Many, though beautiful in their own right, have lost the color and luster of native gold.

In the periodic table of the elements, gold (AU) is centrally located.

Some samples contain gold combined with tellurium or silver, or both. Many gold deposits are associated with silver. More silver occurs in Earth's crust than gold; for every seven parts of gold there are 100 parts of silver.

The mineral electrum contains gold and silver (at least 20 percent by weight) in combination. If the silver content is less, then the mineral is considered native gold.

Some samples combine gold with even less common elements such as antimony, tellurium, palladium, platinum and others. These nonnative gold minerals are less common than the first group.

Montbrayite is very rare. It has only been found in five mines worldwide.

Hydrothermal Cocktail

Some samples contain gold- and quartz-filled veins. Such veins are created when hydrothermal—hot water—fluids from several miles below Earth's surface force themselves through rock. As the gold is deposited, other common minerals such as quartz, calcite and barite are usually precipitated, or drawn out from the fluid, along with the gold. The gold is known as ore and the other minerals are known as gangue (pronounced "gang"). Most crystallized gold samples are recovered from quartz- and calcite-rich veins and lodes.

Gold—the mineral—exists on Earth in veins and placer deposits as native gold. Gold—the metal—is extracted from rocks and other minerals.

"Slickensides" is a term used for rock surfaces that move against each other along a crack or fault. The surfaces become smoothed, grooved and polished.

Some deposits, known as greenstone belts, are areas of ancient, molten rock, greater than 2.5 billion years old. Geologists think there was a major gold-forming event around that time, possibly linked to the collision of ancient continents. Although gold deposits continue to form in active volcanic areas, greenstone belts and their gold deposits no longer form on Earth today.

Although most visible gold exposed at on Earth's surface was found long ago, amazing gold discoveries still happen. The Summitville gold boulder was found by Bob Ellithorpe in October 1975, on the access road to the Summitville mine. While at work for a mining company, Ellithorpe spotted a glinting, yellow streak in a large rock at the side of the road. The 52-kilogram (114-pound) boulder contained an estimated 11 kilograms (29 troy pounds) of gold. FINDING THE GOLD

Visible gold deposits are increasingly rare, so new exploration techniques must rely on the physical and chemical characteristics of gold deposits rather than on eyesight alone. Since most gold deposits formed during three main periods of geologic time—2.8-2.6 billion years ago, 1.5 billion years ago, or 400 million years ago—geologists look for rock formations from these periods to increase their chance of finding gold.

Geologists analyze stream and river sediments for naturally occurring elements that are often associated with gold. The magnetic properties of local rocks can help identify hidden gold because some deposits occur with iron-rich rocks. The most productive, but expensive, means of locating gold ore is to drill and analyze rock cuttings for their gold content.

How do geologists find microscopic gold like this? They sometimes look for substances known as pathfinders—elements or minerals that often occur in the same deposits as gold. Arsenic, for example, is easier to find and more widely dispersed than the gold.

No other mineral compares to gold, but some come close enough to fool the eye. The mineral pyrite, long known as fool's gold, can form as cubes, like gold, and often occurs in copper, lead or zinc deposits. Sometimes—but not always—it appears along with true native gold. The presence of chalcopyrite in rocks, the brassy yellow vein in the second sample, has also fooled gold prospectors in the past.

Natural gold specimens—nuggets, for example—are typically worth much more than the value of the gold by weight. It's not surprising that man-made, forged nuggets, like the sample in the case, surface now and then in an attempt to increase the value of the object.

Many of today's biggest mining operations process ores that contain little, if any, visible gold. To obtain trace amounts of gold, these operations remove massive quantities of rock from a mining site, creating large open-pit or underground mines. Pulverized rock is exposed to chemicals such as cyanide and mercury to extract the gold.

Although most gold is mined as a primary commodity, some gold is a byproduct of mining for other ores, including copper, lead and zinc. Byproduct gold accounts for roughly 15 percent of total world gold production.

Chasing Gold, Then and Now

More than 90 percent of all gold ever used has been mined since 1848, when gold was discovered at Sutter's Mill, California, but the lure of gold has been felt since the beginning of human civilization.

GHANA, BRAZIL AND RUSSIA

Ghana
West Africa was one of the major suppliers of gold to the ancient world. Gold came through Carthage, in present-day Tunisia, as early as 264 BC. Arabian traders dominated the gold market through the Middle Ages. Although many African tribes valued gold as a symbol of status and power, it was seldom used as money. In 1471, Portuguese explorers landed on the African coast near the mouth of the Pra, a river whose bed was filled with deposits of gold. Soon the Portuguese were building forts, castles and a prosperous world trade in gold. The area, in present-day Ghana, became known as the Gold Coast.

Portuguese explorers were dazzled by the abundance of African gold and established a thriving Gold Coast trade.

Brazil
In the late 1600s Portugal was in need of new sources of wealth. Expeditions set out to search the Brazilian interior, and in 1695 explorers discovered gold in many streams and rivers. Enslaved Africans, many from the Gold Coast with experience in placer mining, were used to retrieve Brazilian gold. Over the next century, 1,000 tons of gold were exported from the Brazilian state of Minas Gerais.

Russia
In 1719, Peter the Great, czar of Russia, issued an act permitting Russian gold seekers to pursue wealth for the enrichment of the state. By the time of the California gold rush, Russia was providing more than half of all newly mined gold in the world. After the serfs were freed in the mid-1800s, many of them flocked to the gold-mining areas of eastern Siberia.

Despite harsh winters and mosquito-plagued summers, mining boomed in eastern Siberia.
THE GREAT GOLD RUSHES

The gold rushes of the past few hundred years marked the first time that gold was pursued and acquired by—and for—individuals instead of governments or states.

California—1849
The 1848 discovery of gold at Sutter's Mill in California sparked the greatest gold rush of all time. Hopeful prospectors from around the world, known as forty-niners, streamed to the hills of California. This migration played an important role in settling and developing California.

The forty-niners extracted over $500 million worth of gold from California's 100-mile-long Mother Lode. That amounts to over $10 billion at today's rates. The Mother Lode is the gold-bearing quartz formations east of the Sacramento and San Joaquin rivers and west of the Sierra Nevada. The term "mother lode" has come to mean an abundant source of any kind.

Carpenter James Marshall found a pea-shaped lump of gold in a ditch at Sutter's Mill in California.

The forty-niners extracted gold through panning, sluicing, hydraulic washing/mining and dredging.

Australia—1851
Australian prospector Edward Hammond Hargraves discovered gold in the hills around Bathurst, New South Wales, in 1851. Hargraves was a veteran of the California gold rush and recognized in his homeland similarities to the gold-bearing geology he had seen in California.

Despite the government's efforts to control the flood of eager wealth-seekers, gold fever hit hard. Further discoveries were made from Victoria to Western Australia over the next 43 years. The final strike occurred in the Dundas goldfields in 1894.

Although gold had been discovered earlier in Australia, it was Hargraves' strike in 1851 that ignited the Australian gold rush.

Klondike—1896
From the 1850s to the end of the century, gold strikes were reported in Canada and Alaska. But it was the discovery of gold on the Klondike River in 1896 that sparked the last great gold rush, the most physically challenging of all. It took over a year for news of the strike to reach the public. By the time most of the tens of thousands of hardy "stampeders" made the steep, icy ascents of the Chilkoot and White Passes to reach the goldfields in Yukon Territory, Canada, the claims were already staked. By 1906, mining companies had purchased most individual claims. About 404,000 kilograms (1.1 million troy pounds) have been mined from Yukon placer deposits since 1898—an amount worth more than $5 billion in 2006.

After traveling by steamship to Skagway, Alaska, miners made the arduous 500-mile journey to the goldfields on foot and by boat.

"My feet are sore, my heels are blistered, my legs sore and lame, my hands, neck, shoulders sore and chafed from rope. But boys, don't think I'm discouraged … there is a golden glimmer in the distance." -Fred Dewey, Klondike prospector, 1898

Other Significant Gold Rushes

North Carolina and Georgia

The first documented gold find in the United States was in Cabarrus County, North Carolina, in 1799. Conrad Reed, a farmer's son, brought home a curious, 17-pound yellow "rock." His family used the nugget as a doorstop for three years before it was finally identified as gold. The Reed family began to mine gold the next year. They ran a placer mining operation at first—that is, the gold was in surface deposits—but underground mining began in 1831, after the discovery of a subsurface vein.

Enslaved Africans made up the labor force at the Reed Mine. In some southern mines they were allowed to keep a small amount of the gold they found to buy their freedom.

North Carolina led the United States in gold production until 1848, when gold was discovered in California.

Although gold had been mined in Georgia since the 1500s, first by the Cherokee and later by the Spanish, the rediscovery of gold in the 1820s brought a rush of thousands of prospectors to the region. By 1830, 8,500 grams (273 troy ounces) was being mined every day. The federal government produced the first gold coins at its mint in Dahlonega, Georgia.

Black Hills and Lead, South Dakota

In 1874, two miners, apparently part of Lieutenant Colonel George Armstrong Custer's expeditionary force, discovered gold in the Black Hills. News that Custer's expedition had found gold spread quickly and sparked the Black Hills gold rush.

In 1876, Fred and Moses Manuel, along with Hank Harney and Alex Engh, staked a claim near present-day Lead, South Dakota. They named their claim the Homestake. By the time it closed in December 2001, the Homestake Mine ranked as the oldest continuously operating mine in the United States. It had produced nearly 10% of the country's total gold supply.

The Homestake Mine is the deepest mine in North America. Its 375 miles of tunnels are as deep as 8,000 feet below Earth's surface.

Witwatersrand Goldfields, South Africa

Out on a hunting trip in 1834, Carel Kruger discovered gold in the Witwatersrand region of north-central South Africa. It was not until 1886, however, that gold deposits large enough to be profitable were found. Since then, South Africa has become the largest gold producer in the world.

Of all the gold ever mined, 40 percent has come from South Africa. About 98 percent of that gold came from the deep mines of the Witwatersrand region.

SLUICE BOX

The sluice box is a sloping wooden trough with small boards called riffles across the bottom. Miners pour a mix of water and gravel from the streambed into the sluice. Gravity causes the mix to flow down the sluice over the riffles. The dense gold is trapped in the riffles, and the lighter sediment washes away.

Placer miners use a sluice box to harness gravity and extract gold from stream gravels.

Gold Mining, the Environment, and Human Health and Welfare

Today, the global demand for gold is higher than ever. Mining remains the primary means of satisfying this demand—but it comes at a cost.

Mining has a long history of providing raw materials that are highly valued for their industrial, economic and cultural importance. But there are environmental costs to all mining-and gold mining in particular. The most productive gold mines today contain only trace quantities of microscopic particles of gold in rocks known as ore. Because of the very low gold contents of these ores, most modern mining operations require the excavation of massive quantities of ore from extremely large open-pit surface and underground mines. Hazardous chemicals such as mercury and cyanide-bearing solutions are used to extract their gold, and spills and leakage have occurred at mines around the world. Weathering of ores in these mines can also release sulfuric acid and other chemicals to rivers, streams and groundwater. In many cases, all water leaving a mine must be captured and treated for decades.

Today, new U.S. mines must, by law, commit money for environmental remediation, though in practice, the public often ends up paying many of these costs. Historically, environmental regulation was virtually nonexistent, and patchwork legislation still leaves some U.S. states with more protections than others. And while the U.S. has begun tightening its environmental policies, such restrictions vary widely overseas. Today, much gold mining occurs in developing countries, which depend on mining for income, but where environmental protections may be weak. Cleaner mining practices have been developed to reclaim cyanide used to process ore and prevent acids and other pollutants from reaching groundwater—but they increase costs. While responsible mining techniques prevent much larger cleanup costs later on, cheaper methods will likely remain in use until governments and industry create a consistent set of environmental regulations that is enforced worldwide.

Poor environmental management often comes with deficient labor practices. Both are hazardous to miners, who in many cases include children. There have been some improvements. In recent years several countries have greatly reduced the number of children in mining. Unfortunately, practices that prevent child labor and provide for the health, safety and welfare of miners are very inconsistent, especially among developing countries.

For most of us, gold mining evokes images of the California gold rush: brave, solitary prospectors searching in streams for gold nuggets, or scouring the hills for veins of gleaming metal. But a lot has changed since 1849. Most of the pure, easy-to-reach gold is long gone.

Forming Earth's Gold Deposits

Gold may occur as deposits called lodes, or veins, in fractured rock. It may also be dispersed within Earth's crust. Most lode deposits form when heated fluids circulate through gold-bearing rocks, picking up gold and concentrating it in new locations in the crust. Chemical differences in the fluids and the rocks, as well as physical differences in the rocks, create many different types of lode deposits.

Over millions of years, gold flakes and nuggets worn away from veins are swept into bodies of water. The heavy gold settles in stream-, lake- and riverbeds, and on the sea floor, forming placer ("PLASS-er") deposits.

1. Superheated waters emerge from "smokers," spring-like vents in the seafloor. This occurs where tectonic activity forces the spreading of the oceanic crust. Metal-rich minerals, including small amounts of gold, are deposited as the heated gold-laden water mixes with the cold seawater.

2. Gold-laden water heated by magma-molten rock-in Earth's shallow crust forms a variety of lode gold deposits. Hydrothermal-hot water-fluids rich in sulfur form gold ores in rocks of active volcanoes. The deposits of Summitville, Colorado, are an example.

3. Gold minerals form in hot rocks in and around volcanoes. Low sulfur, gold-bearing hydrothermal fluids form when hot rocks heat ground water. An example of these low-sulfur fluids are hot springs like those at Yellowstone National Park. The ores of Round Mountain, Nevada, are typical low-sulfur deposits.

4. Chemical interactions between hot fluids and sedimentary rocks form deposits of tiny, even invisible, gold particles. The Meikle Mine of Nevada contains such "invisible" gold.

5. Fractures form in Earth's crust as mountains rise. Hydrothermal fluids flow into these spaces and form gold-bearing quartz veins. These fluids are created by hot, deeply buried metamorphic rocks. The Mother Lode and other deposits of the Sierra Nevada gold belt in California are examples.

6. Ancient gold deposits are found in greenstone belts: volcanic belts more than 2.3 billion years old. Although gold deposits continue to form in active volcanic areas, greenstone belts and their gold deposits no longer form on Earth today. Examples include parts of Canada, Zimbabwe and Australia.

7. Placer deposits form at Earth's surface when weathering action exposes gold from other, older lode deposits. The gold is swept into, and settles in, streams, lakes, rivers or the sea floor. Many placer deposits are of recent geologic age, but some are billions of years old.

In most of Earth's crust, gold concentrations are very low. On average, a ton of rock from the crust holds 0.005 grams of gold-compared to 58,000 grams of iron.

World Gold Deposits

Gold has been found on every continent except Antarctica.

It is estimated that worldwide, the total amount of gold ever mined is 152,000 metric tons This is surprisingly small when you consider that this would only fill 60 tractor trailers.

Oceans are the greatest single reservoir of gold at Earth's surface, containing approximately eight times the total quantity of gold mined to date. However, the current cost of extracting it is more than the gold is worth.

World's Largest Deposits of Gold

The massive deposits of the Witwatersrand mines in South Africa have produced more than 40 percent of the world's total production of gold. The origin of these ancient ores—several billion years old—is controversial. Some geologists think these deposits were formed when streams deposited the gold into an ancient lake nearly three billion years ago. Over time, the gold-bearing sediments solidified into rock. Others point to evidence that after an initial placer deposit was formed, the gold was redeposited by hydrothermal fluids, like those in a hot spring.

Treasure of Five Continents

Gold has been mined from most countries on Earth. Although the mineral occurs in many different types of rock and ore deposits, the deposits themselves are surprisingly similar. For example, most gold occurs in native form—that is, pure or nearly pure—and most gold is not visibly crystalline.

Five of Earth's seven continents are represented by the gold in this case.
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