Gold is a chemical element with the symbol Au (from its Latinname aurum) and atomic number 79. It is a highly sought-after precious metal, having been used as money, as a store of value, in jewelry, in sculpture, and for ornamentation since the beginning of recorded history. The metal occurs as nuggets or grains in rocks, underground â€œveinsâ€ and in alluvial deposits. Gold is dense, soft, shiny and the most malleable and ductile substance known. Pure gold has a bright yellow color traditionally considered attractive. It is one of the coinage metals and formed the basis for the gold standard used before the collapse of the Bretton Woods system in 1971.
Modern industrial uses include dentistry and electronics.
Gold dissolves in mercury, forming amalgam alloys, but does not react with it. Gold is insoluble in nitric acid, which will dissolve silver and base metals, and is the basis of the gold refining technique known as â€œinquartation and parting.â€ Nitric acid has long been used to confirm the presence of gold in items, and this is the origin of the colloquial term â€œacid test,â€ referring to a gold standard test for genuine value.
A single gram of gold can be beaten into a sheet of one square meter, an ounce into 300 square feet. Gold leaf can be beaten thin enough to become translucent. 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 caratage, typically 22k, 18k, 14k or 10k, are made, with other metals. Copper is the most common base metal, yielding a redder color. 18 carat gold containing 25% copper is found in antique and Russian jewelry and has a distinct copper cast, itâ€™s called rose gold. 14 carat gold-copper alloy is nearly the same color as some bronze alloys, and both are used to produce police and other badges. Blue gold can be made by alloying with iron. Blue gold is more brittle and therefore more difficult to work with when making jewelry. 14 and 18 carat gold alloys with silver alone appear greenish-yellow and are referred to as green gold.
The Japanese craft of Mokume-gane exploits the color contrasts between laminated colored gold alloys to produce decorative wood-grain effects.