Gold has been widely used throughout the world as a monetary exchange for centuries, the gold coins or other substances through gold-convertible paper instruments by establishing gold standards in which the total value is represented in a store of gold reserves.
However, production has not grown in relevant to the world's economies. Today, the output of gold mining is declining. With the sharp growth of economies in the 20th century, and the increasing of foreign exchange, the world's gold reserves and their trading market have become a small fraction of all markets and fixed exchange rates of currencies to gold were no longer sustained. After World War II gold was replaced by a system of convertible currency. Gold standards and the direct convertibility of currencies to gold have been abandoned by world governments, being replaced by flat currency in their stead. Switzerland was the last country to tie its currency to gold.
Pure gold is too soft for day-to-day monetary use and is typically hardened by alloying with copper, silver or other base metals. The gold content of alloys is measured in carats (k). Pure gold is designated as 24k and a standard 22k alloy called crown gold, for hardness (American gold coins for circulation after 1837 contained the slightly lower amount of 0.900 fine gold).
Investment - Many gold collectors store the gold coins as an investment tool as against inflation or other economic disruptions. However, some economists do not believe it at the same theory. The modern gold coins for investment or collector purposes do not require good mechanical wear properties; they are typically fine gold at 24k, although the American Gold Eagle, the British gold sovereign, and the South African Krugerrand continue to be minted in 22k metal in historical tradition. The special issue Canadian Gold Maple Leaf coin contains the highest purity gold of any gold coin, at 99.999%, while the popular issue of Canadian Gold Maple Leaf coin has a purity of 99.99%. Several other 99.99% pure gold coins are available. In 2006, the United States Mint began to produce the American Buffalo gold coin with a purity of 99.99%. The Australian Gold Kangaroos were the first coin launched in 1986 as the Australian Gold Nugget but changed the reverse design in 1989. Other popular modern coins include the Austrian Vienna Philharmonic gold coin and the Chinese Gold Panda.
Jewelry - Because of the softness of pure gold, it is usually alloyed with base metals for use in jewelry, altering its hardness, melting point, color and other properties. Alloys with lower carat contain higher percentages of copper or other base metals or silver or palladium in the alloy. Copper is the most commonly used as base metal, yielding a reddish color. Eighteen-carat gold containing 25% of copper is mostly found in antique and Russian jewelry, copper cast which can create rose gold. Fourteen-carat of gold-copper alloy is nearly identical as 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 specialize jewelry. Blue gold is more brittle and therefore more difficult to work with jewelry. Fourteen and eighteen carat 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-carat gold containing 17.3% nickel, 5.5% zinc and 2.2% copper is silvery in appearance. 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-carat white gold alloys are far more resistant to corrosion than are either pure silver or sterling silver.