It's a pirate's booty and an ingredient in microcircuits. It's been used to make jewelry since at least 4000 B.C. and to treat cancer only in recent decades. It's in the pot at the end of the rainbow and in the coating on astronaut visors.
What's this element that bridges old and new — and myth and science — so seamlessly? Gold.
Gold, the 79th element on the Periodic Table of the Elements, is one of the more recognizable of the bunch.
Just the facts
- Atomic Number (number of protons in the nucleus): 79
- Atomic Symbol (on the Periodic Table of Elements): Au
- Atomic Weight (average mass of the atom): 196.9665
- Density: 19.3 grams per cubic centimeter
- Phase at Room Temperature: Solid
- Melting Point: 1,947.7 degrees Fahrenheit (1,064.18 degrees C)
- Boiling Point: 5,162 degrees F (2,850 degrees C)
- Number of isotopes (atoms of the same element with a different number of neutrons): Between 18 and 59, depending on where the line for an isotope is drawn. Many artificially created gold isotopes are stable for microseconds or milliseconds before decaying into other elements. One stable isotope.
- Most common isotopes: Au-197, which makes up 100 percent of naturally occurring gold.
Shiny star matter
Humans have been decorating themselves with gold since at least 4000 B.C., according to the National Mining Association. From Eastern Europe to the Middle East to the tombs of Egyptian Pharaohs, gold appears throughout the ancient world. A Stone Age woman found buried outside of London wore a strand of gold around her neck; Celts in the third century B.C. wore gold dental implants; a Chinese king who died in 128 B.C. was buried with gold-gilded chariots and thousands of other precious objects.
Gold is malleable and shiny, making it a good metalworking material. Chemically speaking, gold is a transition metal. Transition metals are unique, because they can bond with other elements using not just their outermost shell of electrons (the negatively charged particles that whirl around the nucleus), but also the outermost two shells. This happens because the large number of electrons in transition metals interferes with the usual orderly sorting of electrons into shells around the nucleus.
All the gold that makes up earrings and cufflinks and electronics components today originated in space: According to a 2011 paper in the journal Nature, a meteor bombardment nearly 4 billion years ago brought 20 billion billion tons of a gold-and-precious-metal-rich space rock to Earth. Tracing gold's origin back even further takes us into deep space. A 2013 study in The Astrophysical Journal Letters found that all of the gold in the universe was likely birthed during the collisions of dead stars known as neutron stars.
Veins of gold mined from the earth are the result of hot fluids flowing through gold-bearing rock, picking up gold and concentrating it in fractures, according to the American Museum of Natural History (AMNH).
What is a karat?
Most gold jewelry isn’t made of pure gold. The amount of gold in a necklace or ring is measured on the karat scale. Pure gold is 24 karats. Bars of gold kept in Fort Knox and elsewhere around the world are considered to be 99.95 percent pure, 24-karat gold.
As metals are added to gold during jewelry making, the gold becomes less fine and the number of karats drops. For example, 12 karat gold contains 50 percent gold and 50 percent alloys by weight.
The word karat comes from the carob seed. In ancient Asian bazaars, the seeds were used to balance scales that measured the weight of gold.
How much gold is in Fort Knox?
To keep up with the country’s mounting gold reserves, the United States Bullion Depository opened at the Fort Knox U.S. Army Garrison in Kentucky in 1937. The first shipment of gold arrived from Philadelphia in trains surrounded by military troops.
Fort Knox is framed in steel with walls of concrete.
Despite the defense of a 20-ton steel door, a dirty rumor in the 1970s suggested that the gold in Fort Knox was gone. To quell people’s fears, the director of the United States Mint guided congressmen and journalists through one room of the vault, and its 8-foot-tall stacks of 36,236 bars of gold.
Each bar weighs 400 troy ounces according to the U. S. Department of Treasury. One troy ounce equals about 1.1 avoirdupois ounces. The entire stockpile now weighs 147.3 million troy ounces, which is worth about $130 billion at today's prices.
The inferior mineral nicknamed fool’s gold only mimics gold in looks. Pyrite is more common, harder, and more brittle than gold. When crushed into powder, it looks greenish-black, whereas real gold powder is yellow.
Pyrite contains sulfur and iron. During World War II it was mined to produce sulfuric acid, an industrial chemical. Today, it is used in car batteries, appliances, jewelry, and machinery.
Although fool’s gold can be a disappointing find, it is often discovered near sources of copper and gold. A miner who stops digging once they have a piece of pyrite in hand is the real fool.
- Two-thirds of the world's gold is mined in South Africa, according to Lawrence Livermore National Laboratory.
- Seventy-eight percent of the world's yearly supply of gold is used in jewelry, according to the AMNH. The rest goes to electronics and dental and medical uses.
- The atomic symbol of gold, Au, comes from the Latin word for gold, aurum.
- Astronaut helmets come equipped with a visor coated with a thin layer of gold. The gold blocks harmful ultraviolet rays from the sun.
- The world's largest gold crystal is the size of a golf ball and comes from Venezuela. The 7.7-ounce (217.78 grams) crystal is worth about $1.5 million.
- Earthquakes can create gold: A 2013 study in the journal Nature Geoscience found that during earthquakes, water in faults and fractures vaporizes, leaving gold behind.
- The first purely gold coins were manufactured in the Asia Minor kingdom of Lydia in 560 B.C., according to the National Mining Association.
- Gold has a number of artificial, unstable isotopes (the exact number depends on the scientist you consult), but occurs naturally only as Au-197.
- You can eat gold … if you really want to. Gourmet shops sell edible gold leaf and flakes that add glitter to everything from pastries to vodka to olive oil. Don't fear for your stomach: The gold isn't digested and just passes right through, according to Edible Gold, a company that sells gold leaf.
Gold is still used in jewelry, of course, but this element has also gone high-tech. Gold is an excellent conductor of electricity and is very non-reactive with air, water and most other substances, meaning it won't corrode or tarnish.
Gold is also used in medicine. The radioactive gold isotope Au-198 can be injected directly into the site of a tumor, where its radiation can destroy tumor cells without much spillover to the rest of the body. In 2012, researchers reported in the journal Proceedings of the National Academy of Sciences that they could link nanoparticles of Au-198 with a compound found in tea leaves to treat prostate cancer. The tea compound is attracted to the tumor cells, keeping the nanoparticles glued to the right spot for several weeks while the radiation treatment occurs. (The method has yet to be tested on humans.)
In some cases, gold nanoparticles are the only way a drug can work. The anti-cancer drug TNF-alpha kills cancer very effectively. Unfortunately, it's also incredibly toxic to healthy cells. However, clinical trials now underway have found that linking TNF-alpha drugs to gold nanoparticles can successfully treat tumors, because the drugs hit their targets directly, according to Benchmarks, an online publication of the National Cancer Institute.
There's just one problem with humanity's continued love affair with gold: Getting it out of the ground. About 83 percent of the 2,700 tons of gold mined each year is extracted using a process called gold cyanidation, said Zhichang Liu, a postdoctoral researcher in chemistry at Northwestern University in Illinois. This process uses cyanide to leach gold out of the rock that holds it. Unfortunately, cyanide is toxic, and the process is anything but environmentally friendly.
There could be hope for lovers of gold baubles (and electronic circuits and nanomedicine), however. In 2013 Liu and his colleagues reported in the journal Nature Communications that they'd stumbled upon a way to extract gold from ore with benign starch rather than toxic cyanide.
"Actually, we found this method by accident," Liu told Live Science. While trying to fabricate a porous material, the researchers mixed a starch called alpha-Cyclodextrin with gold salts (charged molecules of gold). To their surprise, the gold precipitated out of the solution rapidly.
The team has patented the method, which easily extracts gold at more than 97 percent purity in one step, Liu said. They're now working with investors to scale up the process.
"Hopefully, we can find a nice, green way to replace the cyanidation process," Liu said.
Learn more about gold:
- Learn about the Gold Rush of 1849
- What Is the Gold Standard?
- Gold Mining Around the World (with Interactive Map)
- The Current Price of Gold (and Other Metals)
A century ago a lucky gold miner might find a nugget as big as an apple. That gold is gone. Now mines squeeze precious flecks from deposits that look to the naked eye to be just plain old rock.
Gold Quiz: From Nuggets to Flecks