Gold is a chemical element with the symbol Au and the atomic number 79. In its purest form, it is a bright, slightly reddish yellow, dense, soft, malleable and ductile metal. Gold's atomic number of 79 makes it one of the higher atomic number elements that occurs naturally in the universe. It is thought to be produced in supernova nucleosynthesis and from the collision of neutron stars, as well as to have been present in the dust from which the solar system was formed.
Gold is a precious metal, used for coinage, jewellery & other arts recorded throughout history. In the past, a gold standard was often implemented as a monetary policy within and between nations. Gold coins ceased to be minted as circulating currency in the 1930's. The historical value of gold was rooted in its relative rarity, easy handling and minting, easy smelting and fabrication, resistance to corrosion & chemical reasons, and distinctive colour.
Gold is the most malleable of all metals; a single gram can be beaten into a sheet of 1 square meter, and an ounce into 300 square feet. Gold leaf can be beaten thin enough to become transparent. The transmitted light appears greenish blue, because gold strongly reflects yellow and red.
Because of the softness of pure (24k) gold, it is usually alloyed with base metals for use in jewelry, altering its hardness and ductility, melting point, color and other properties. Alloys with lower karat rating, typically 22k, 18k, 14k or 10k, contain higher percentages of copper or other base metals or silver or palladium in the alloy. Copper is the most commonly used base metal, yielding a redder color.
Eighteen-karat gold containing 25% copper is found in antique and Russian jewelry and has a distinct, though not dominant, copper cast, creating rose gold. Fourteen-karat gold-copper alloy is nearly identical in color to certain bronze alloys, and both may be used to produce police and other badges. Blue gold can be made by alloying with iron and purple gold can be made by alloying with aluminium, although rarely done except in specialized jewelry. Blue gold is more brittle and therefore more difficult to work with when making jewelry.
Fourteen- and eighteen-karat gold alloys with silver alone appear greenish-yellow and are referred to as green gold. White gold alloys can be made with palladium or nickel. White 18-karat gold containing 17.3% nickel, 5.5% zinc and 2.2% copper is silvery in appearance. Nickel is toxic, however, and its release from nickel white gold is controlled by legislation in Europe.
Alternative white gold alloys are available based on palladium, silver and other white metals,but the palladium alloys are more expensive than those using nickel. High-karat white gold alloys are far more resistant to corrosion than are either pure silver or sterling silver.