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Gold is called a 'noble' metal because it does not oxidize under ordinary conditions. Its chemical symbol Au is derived from the Latin word 'aurum.' In pure form gold has a metallic luster and is sun yellow, but mixtures of other metals, such as silver, copper, nickel, platinum, palladium, tellurium, and iron, with gold create various color hues ranging from silver-white to green and orange-red. Pure gold is relatively soft--it has about the hardness of a penny. It is the most malleable and ductile of metals. The specific gravity or density of pure gold is 19.3 compared to 14.0 for mercury and 11.4 for lead. Impure gold, as it commonly occurs in deposits, has a density of 16 to 18, whereas the associated waste rock (gangue) has a density of about 2.5. The difference in density enables gold to be concentrated by gravity and permits the separation of gold from clay, silt, sand, and gravel by various agitating and collecting devices such as the gold pan, rocker, and sluicebox.
Mercury (quicksilver) has a chemical affinity for gold. When mercury is added to gold-bearing material, the two metals form an amalgam. Mercury is later separated from amalgam by retorting. Extraction of gold and other precious metals from their ores by treatment with mercury is called amalgamation. Gold dissolves in aqua regia, a mixture of hydrochloric and nitric acids, and in sodium or potassium cyanide. The latter solvent is the basis for the cyanide process that is used to recover gold from low-grade ore.
The degree of purity of native gold, bullion (bars or ingots of unrefined gold), and refined gold is stated in terms of gold content. 'Fineness' defines gold content in parts per thousand. For example, a gold nugget containing 885 parts of pure gold and 115 parts of other metals, such as silver and copper, would be considered 885-fine. 'Karat' indicates the proportion of solid gold in an alloy based on a total of 24 parts. Thus, 14-karat (14K) gold indicates a composition of 14 parts of gold and 10 parts of other metals. Incidentally, 14K gold is commonly used in jewelry manufacture. 'Karat' should not be confused with 'carat,' a unit of weight used for precious stones. The basic unit of weight used in dealing with gold is the troy ounce. One troy ounce is equivalent to 20 troy pennyweights. In the jewelry industry, the common unit of measure is the pennyweight (dwt.) which is equivalent to 1.555 grams.
USGS Web Site