Gold is almost indestructible and has been used and then reused for centuries to the extent that all gold of known existence is almost equal to all the gold that has ever been mined. Gold is a great medium metal for jewelry, as it never tarnishes. In addition to its softness, it is both the most malleable and most ductile of all elements. This means that it can be hammered into extremely thin sheets (approaching a small number of atoms) and can be drawn into extremely fine wire. Gold in the form of very thin sheets, called gold leaf, has many decorative uses. Elemental gold is an excellent conductor of electricity and heat, surpassed only by the other members of group 1B, copper and silver. Gold usually forms compounds (and complexes) by giving up either one of three of its valence electrons. It is commonly alloyed with other metals, as in jewelry, in proportions that yield desired hardiness and colors, (see metal section). An alloy of gold, silver, and copper, in which the amounts of silver predominates, is called "green gold." An alloy of the same three elements in which copper predominates is called "red gold." An alloy of gold and nickel is called "white gold." The purity of alloyed gold is expressed by the karat system, where the percent of gold by weight is given as a fraction of 24. Therefore, pure gold is 24 karat, whereas 18 karat gold is 18/24, or 75%, gold by weight. Gold dissolves in very few solvents, among them aqua regia and the various solutions of cyanide that are used in ore extraction. When gold does dissolve, it is generally by forming complexes. Gold also forms amalgams with mercury and as such is used in dentistry.
The principal source of gold in primitive times was undoubtedly stream placers, although there is considerable evidence in certain gold belts (e.g., Egypt and India) that eluvial deposits, auriferous gossans, and the near surface parts of friable (oxidized) veins were mined. The eluvial and alluvial placers were worked in the crudest manner by panning or the simplest form of sluicing. Gold is obtained by two principal mining methods; placer and vein mining, and also as a by product of the mining of other metals. Placer mining is used when the metal is found in unconsolidated deposits of sand and gravel from which gold can be easily separated due to its high density. The sand and gravel are suspended in moving water; the much heavier metal sinks to the bottom and is separated by hand. The simplest method, called panning, is to swirl the mixture in a pan rapidly enough to carry the water and most of the gravel and sand over the edge while the gold remains on the bottom. Panning is the classic method used by the old prospectors and is immortalized in story, art, and song.
One of the principal uses of gold today is as a currency reserve. Gold was for centuries used directly as currency along with silver. During the 19th century it assumed a role as the sole basis of the currencies of most nations; paper money was directly convertible into gold. World War I disrupted this system. The original gold standard was gradually abandoned (the United States stopped minting gold coinage in 1934), and the dollar eventually emerged as the principal unit of international monetary transactions . Since the 1970s gold has been bought and sold on the market, with widely fluctuating prices, and gold reserves maintain only a very indirect relationship with the values of currencies. There is a large and rapidly growing demand for gold in industrial processes. Its relatively high electrical conductivity and extremely high resistance to corrosion make the metal critically important in micro electrical circuits. Minute quantities dissolved in glass or plastic sheets prevent the passage of infrared radiation and make an efficient heat shield.Because of its chemical stability, gold is in demand for bearing used in corrosive atmospheres. It is also plated on surfaces exposed to corrosive fluids or vapours. Its lack of toxicity and its compatibility with living systems make it indispensable in dentistry and medicine, and its beauty has made it outstanding in the arts and crafts since ancient times.