Pure Gold and Yellow Gold jewellery
Source: Australia, Argentina, Canada, Chile, India, Russia, South Africa, USA
Pure 24 carat gold is a highly malleable, ductile, and soft metal with a deep yellow to orange-yellow color. Other colors of gold are created by alloying pure gold with other metals. Gold as a precious metal is a chemical element (noble metal) in the periodic table with the symbol Au (from the Latin word "aurum"), and its atomic number 79.
Gold has been valued as a precious commodity for thousands of years, and it may have been one of the first metals used as ornamentation, or in rituals. Gold objects found in Spanish caves date back to the late Paleolithic period (c.40,000 BC). In 2600 BC, gold was described as being "more plentiful than dirt" in hieroglyphs from Mesopotamia and Egypt. One of the earliest known maps is the Turin papyrus (c.1160 BC) found in Deir el-Medina, Thebes, which shows the plan of a gold mine in Nubia along with specifics of the geology and topography.
 Mask of Agamemnon (c.1500 BC) Gold in Ancient Egypt (c.1340 BC) |
Virtually every ancient civilization from the Mycenae and Romans to the Incas, Aztecs and ancientAsiatic cultures have all used gold as jewelry, currency, ritualistic ornamentation (ancient Egyptian and Chinese funerary items), a symbolic metaphor (the Old Testament), or a mythological icon such as Jason's fabled golden fleece.
Gold Distribution & Occurrence
Occasionally large accumulations of "native gold" (typically 8-10% silver) nuggets occur (above, right), but typically gold occurs as minute grains, or within ores. Gold grains can occur between mineral grain boundaries, or as inclusions within minerals. Gold is widely distributed in the Earth's crust and can be broken into two categories:
- Reef Gold: Extracted by hard-rock or sub-surface mining.
- Placer Gold: Surface gold found in alluvial or placer deposits.
Reef gold is created by hydrothermal activity, and these ore deposits can occur within metamorphic rocks and igneous rocks. Reef gold is mined by "hard rock" or "sub-surface" mining.
Ores (aka "lode deposits") that contain native gold have microscopic particles embedded in the rock, which can have veins of quartz or sulfide minerals like pyrite, calaverite, krennerite, petzite, pyrhotite, galena, sylvanite, and stibnite. Native gold is also found as free flakes, grains or nuggets (free gold) that have been eroded from rocks and redistributed in alluvial, or placer deposits.
 Kalgoorlie Gold Mine in Australia (photo: public domain) |  Gold Ore (photo: US Gov) |
Free gold is typically richer at the surface of gold-bearing veins due to the degradation of accompanying minerals from oxidation or weathering. Gold nuggets can be formed out of particles or grains by intense water activity.
Gold Metallurgy
Gold, caesium (Cs) and copper (Cu) are the only elemental metals with a natural color other than gray or white. Gold used in jewelry is measured in karats (k), with pure gold being 24 karat. 24 karat pure gold is always yellow in color. Pure gold is considered to be too soft for most jewelry so it is more commonly sold in lower measurements of 20k, 18k, and 14k.
- 24k: 100% pure gold
- 18k: 75% pure gold, mixed with other alloys
- 14k: 58.3% pure gold, mixed with other alloys
A lower "k" or "carat" number indicates a higher percent of copper or silver mixed into the alloy. Copper is more commonly used than silver to dilute the concentration of pure 24k gold.
 |  Gold Nugget (photo: US Gov) |
The adjective "auric" refers to something being made of gold. Gold does not react with most chemicals but can be attacked by corrosive agents such as aqua regia, chlorine, and fluorine. Gold is insoluble in nitric acid, which has been used to confirm the presence and purity of gold objects (the "acid test") because nitric acid can dissolve silver or base metals.
Gold can be deformed plastically without fracture, due to its high ductility, but its malleability makes the pure metal too soft for most jewellery. Working, or "annealing" a metal such as gold can make it both harder, and stronger. However, there is a point at which a metal can be overworked, which will eventually cause it to easily fracture. Because of its ductility and malleability, gold can be drawn into very thin wire, or pounded into paper-thin sheets of gold leaf.
 Gold Coins (photo: public domain) |  999.9 Pure gold bullion & nuggets |
Gold solder is used for joining parts in gold jewelry, but if the work is to be of hallmarking quality, the gold solder must match the carat weight of the individual parts.
Gold Pricing
The price of gold is determined on the open market (spot market), by a procedure known as "gold fixing." Originating in London around 1919, gold fixing provides a twice-daily benchmark figure to the industry.
Like all precious metals, gold is measured by grams or troy weight. When it is alloyed with other metals the term "carat" or "karat" is used to indicate the amount of gold present, with 24 Carats being pure gold, and lower ratings proportionally less. The purity of a gold bar (gold bullion) can also be expressed as a decimal figure ranging from ".0" (no gold) to .9999 (totally pure), which is known as its "millesimal fineness."
Traditionally, gold coins that were intended for circulation have typically been made of a 22k alloy called "crown gold," which increased their hardness over pure gold. While the American Gold Eagle, American Buffalo, British Gold Sovereign and Krugerrand continue to be made of 22k gold, the Canadian Gold Maple Leaf has a purity of 99.99 percent.
There is also a "special issue" Canadian Gold Maple Leaf coin and the Australian Gold Nugget which both contain the highest purity gold of any bullion coins, at 99.999 percent, or .9999 millesimal fineness. The ISO currency code of gold bullion is XAU.
Physical Properties of Gold
| Name, Atomic Symbol, # | Gold, Au, 79 |
| Element Category | transition metals (Period 6) |
| Crystal Structure | cubic, face centered |
| Specific gravity (SG) | 19.32 |
| Mohs Hardness Scale | 2.5 |
| Vickers Hardness (VHN or HV) | 216 MPa |
| Melting Point | 1947.52F (1064.18C, 1337.33K) |
| Boiling Point | 5173F (2856C, 3129K) |
| Magnetic Ordering | diamagnetic |
| Chemical Composition | Noble Metal, Auric Chloride (AuCl3), Chlorauric Acid (HAuCl4) |
Conflict Gold
In a situation that is similar to conflict diamonds, gold has become a source of conflict in the war-torn regions of central Africa. In the easter part of the Democratic Republic of Congo (DRC), artisanal miners are fueling an estimated production of $50 million per year, with the most of the money being used to fund paramilitary operations and insurections.
Mines such as the Kaniola mine in South Kivu province produce most of Congo's gold which is then smuggled into neighboring Uganda, and on to processing facilities in developed contries. Jewelry industry trade groups such as CIBJO (The World Jewellery Confederation) are attempting to source gold using similar methods employed by the Kimberley Process and the KPCS certification scheme.
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