Gold (Au, Atomic Number 79) is a transition metal, characterized by its distinctively bright, reddish-yellow color when seen in its purest form. It is well known for its low reactivity properties, and is found in the solid state on earth under most normal conditions of temperature and pressure. Pure gold is highly malleable and ductile, properties which have argubaly made it even more alluring than even its rarity. In fact, a single gram of gold can be beaten to produce a gold sheet of one square meter. It is unaffected by most acids and bases, but is soluble in aqua regia and alkaline solutions containing cyanide. The metal readily dissolves in mercury to form an amalgam at room temperature. At higher temperatures, Gold will form alloys with other metals. The melting point of gold is 1,947.7 degrees Fahrenheit (1,337K, 1064oC), and boiling its point is 5,162 degrees Fahrenheit (3,123K, 2,850oC). Gold is also a good conductor of heat and electricity.
Due to its low reactivity properties, gold occurs in rocks and alluvial deposits, with its native form usually taking shape as larger nuggets or smaller grains of gold. Besides subterranean deposits, trace concentrations of gold are also to be found within many other substances in nature, such as dissolved in seawater (at 0.012 parts per billion) and freshwater. As a whole, the Oceans are a huge reservoir of gold deposits, but the cost of extracting gold from the ocean bed is well over its market value. On the mainland, Gold deposits are found in many countries across the world. Some of the largest deposits of this valuable metal have been found in the Witwatersrand mines in South Africa. Other countries with considerable gold deposits include China, the US, Russia, Peru, Australia, and Canada, among others.
Although gold occurs in distinct geological settings with different types of deposits, the basic classification of gold deposits can be placed into primary and secondary types. The primary deposits are those formed when gold precipitates during chemical reactions within the Earth’s crust. These reactions occur between hot, mineralizing fluids and rocks in the crust. The formation of these hydrothermal deposits of gold can be either epigenetic or syngenitic, depending on whether the deposits of gold form after the formation of surrounding rocks, or during the formation of these rocks, respectively. Secondary deposits form as a result of mechanical (e.g. erosion and weathering) or chemical alterations to the primary deposits.
The most notable uses for gold are those involved in the creation of ornamental jewelry. The malleability and ductility of gold makes it easy to carve intricate designs using this metal. The golden appearance and lustre of this metal increases its attractiveness, and the fact that it is non-reactive makes it an ideal metal for long-term wear as jewelry. Because of the high value associated with gold, this precious metal is often used for awards and as a status symbol indicator in societies across the globe. Due to its status of grandeur, gold is also often used as a medium of investment and exchange. Its high price and limited supply endows it with excellent investment value. Sophisticated electronic equipment, such as expensive watches, often have gold found in their circuits to facilitate durability. This metal is also used in the aerospace industry to create circuitry and other devices. Gold, due to its non-reactive nature, is harnessed in dentistry, molded into fillings, crowns, and other dental applications. Some medical procedures also involve gold, such as the use of radioactive gold isotopes in cancer patients, or use of gold in treating rheumatoid arthritis.
Gold is a highly expensive commodity whose price is measured by troy weight and grams. The purity of gold is also measured in karats when it exists in the form of alloys. In times past, South Africa had for long led the world in gold production. For example, about 79% of the entire global production of gold was accounted for by this single country in 1970. However, in 2007, China captured the leading position, replacing South Africa for the first time since 1905. According to 2014 statistics, China, Australia, the United States, and Russia occupy the top four positions in gold production today. The procedures applied to extracting gold depends on the natures of the gold deposits themselves. Oxide ore deposits are extracted by first shattering the rocks with explosives, and then extracting gold by "cyanidation", which takes gold from ore and put it into a water-soluble state using cyanide. Alluvial deposits of gold are mined using high performance hydraulic hoses, which simply wash the surrounding earth away in a process that is essentially rapid, manmade erosion.