As the metal
Medium of monetary exchange
In various countries, gold is used as a standard for monetary exchange, in coinage and in jewelry. Pure gold is too soft for ordinary use and is typically hardened by alloying with
copper or other base metals. The gold content of gold alloys is measured in carats (k), pure gold being designated as 24k.
Gold coins intended for circulation from 1526 into the 1930s were typically a standard 22k alloy called crown gold, for hardness. Modern collector/investment bullion coins (which do not require good mechanical wear properties) are typically 24k, although the American Gold Eagle and British
gold sovereign continue to be made at 22k, on historical tradition. The Canadian Gold Maple Leaf coin contains the highest purity gold of any popular bullion coin, at 99.999% (.99999 fine). Several other 99.99% pure gold coins are currently available, including Australia's Gold Kangaroos (first appearing in 1986 as the Australian Gold Nugget, with the kangaroo theme appearing in 1989), the several coins of the Australian Lunar Calendar series, and the Austrian Philharmonic. In 2006, the U.S. Mint began production of the American Buffalo gold bullion coin also at 99.99% purity.
Today, gold has fallen out of use in coins made for general circulation.
base metals in the alloy. Copper is the most commonly used base metal, yielding a redder metal. Eighteen carat gold containing 25% copper is found in antique and Russian jewelry and has a distinct, though not dominant, copper cast, creating rose gold. Fourteen carat 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 aluminum, 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 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 silver 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 (World Gold Council), 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. The Japanese craft of Mokume-gane exploits the colour contrasts between laminated colored gold alloys to produce decorative wood-grain effects.
gold is a gold and copper alloy widely used for specialized jewelry due to its reddish colour. It is also known as pink gold and red gold. Because it was popular in Russia at the beginning of the nineteenth century it is also known as Russian gold, although this term has become somewhat rare. Since it is an alloy, there is no such thing as "pure rose gold"; pure gold is yellow and pure copper is reddish. Although the names are often used interchangeably, the difference between red, rose, and pink gold is the copper content: The higher the copper content, the stronger the red coloration. A common alloy for rose gold is three-quarters gold and one-quarter copper (18K).
Rose gold is also the name of a commonly-used finish in bell interiors of musical instruments. It consists of a very thin plating of copper over a layer of silver. No actual gold is used. The highest karat version of rose gold is also known as crown gold, which is 22 karat. 18 karat rose gold may be made of 25% copper and 75% gold. For 18 karat rose gold, typically about 4% silver is added to the 75% gold and 21% copper to give a rose color. 14 karat red gold is often found in the Middle East and contains 41.67% copper.
copper or other base metals. The gold content of gold alloys is measured in carats (k), pure gold being designated as 24k.Gold coins intended for circulation from 1526 into the 1930s were typically a standard 22k alloy called crown gold, for hardness. Modern collector/investment bullion coins (which do not require good mechanical wear properties) are typically 24k, although the American Gold Eagle and British
gold sovereign continue to be made at 22k, on historical tradition. The Canadian Gold Maple Leaf coin contains the highest purity gold of any popular bullion coin, at 99.999% (.99999 fine). Several other 99.99% pure gold coins are currently available, including Australia's Gold Kangaroos (first appearing in 1986 as the Australian Gold Nugget, with the kangaroo theme appearing in 1989), the several coins of the Australian Lunar Calendar series, and the Austrian Philharmonic. In 2006, the U.S. Mint began production of the American Buffalo gold bullion coin also at 99.99% purity.Today, gold has fallen out of use in coins made for general circulation.
Jewelry
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 "k", typically 22k, 18k, 14k or 10k, contain higher percentages of copper, silver or other
base metals in the alloy. Copper is the most commonly used base metal, yielding a redder metal. Eighteen carat gold containing 25% copper is found in antique and Russian jewelry and has a distinct, though not dominant, copper cast, creating rose gold. Fourteen carat 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 aluminum, 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 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 silver 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 (World Gold Council), 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. The Japanese craft of Mokume-gane exploits the colour contrasts between laminated colored gold alloys to produce decorative wood-grain effects.Rose gold
Rose
gold is a gold and copper alloy widely used for specialized jewelry due to its reddish colour. It is also known as pink gold and red gold. Because it was popular in Russia at the beginning of the nineteenth century it is also known as Russian gold, although this term has become somewhat rare. Since it is an alloy, there is no such thing as "pure rose gold"; pure gold is yellow and pure copper is reddish. Although the names are often used interchangeably, the difference between red, rose, and pink gold is the copper content: The higher the copper content, the stronger the red coloration. A common alloy for rose gold is three-quarters gold and one-quarter copper (18K).Rose gold is also the name of a commonly-used finish in bell interiors of musical instruments. It consists of a very thin plating of copper over a layer of silver. No actual gold is used. The highest karat version of rose gold is also known as crown gold, which is 22 karat. 18 karat rose gold may be made of 25% copper and 75% gold. For 18 karat rose gold, typically about 4% silver is added to the 75% gold and 21% copper to give a rose color. 14 karat red gold is often found in the Middle East and contains 41.67% copper.
