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A gold coin is a coin that is made mostly or entirely of gold. Most gold coins minted since 1800 are 900–920 ‰ gold (22karat), while most of today's gold bullion coins are pure gold, such as the Britannia, Canadian Maple Leaf, and American Buffalo. Alloyed gold coins, like the American Gold Eagle and South African Krugerrand, are typically 917 ‰ gold by weight, with the remainder being silver and copper.
Until about the 1930s, gold coins were circulation coins, including Exonumia and Gold dinar. Since then, gold coins have mainly been produced as for investors and as commemorative coins for Coin collecting. While modern gold coins are still legal tender, they are not used in everyday financial transactions, as the metal value invariably exceeds the nominal value. For example, the quarter-ounce American Gold Eagle has a denomination of $10, but a metal value of approximately $500 (as of ).
The of are dominated by , but gold coins may occasionally contribute.
Gold has been used as money for many reasons. It is fungible, with a low difference between the prices to buy and sell. Gold is easily transportable, as it has a high value-to-weight ratio compared to other precious metals such as silver. Gold can be re-coined, divided into smaller units, or melted into larger units such as gold bars, without destroying its metal value. The density of gold is higher than most other metals, making it difficult to pass . Additionally, gold is extremely unreactive chemically: it does not tarnish or corrode over time.
In 2012 the Royal Canadian Mint produced the first gold coin containing a 0.11–0.14 ct diamond. The Queen's Diamond Jubilee coin was crafted in 99.999% pure gold with a face value of $300.
The largest legal-tender gold coin ever produced was unveiled in 2012 by the Perth Mint in Western Australia. Known as the "1 Tonne Gold Kangaroo Coin" and with a face value of one million dollars, it contains one tonne of 9999 pure gold and is approximately 80 cm in diameter by 12 cm thick.
Bullion coin counterfeits were formerly rare and fairly easy to detect when comparing their weights, colors and sizes to authentic pieces. This is because the cost of reproducing a coin precisely can exceed the market value of the originals. However, since about 2015 counterfeit coins have been "flooding the market at an astonishing rate" and "it's gotten to the point where even people who deal with coins all the time may not be able to recognize a counterfeit coin right away" (American Numismatic Association (ANA), 2016). The coins consist mostly of tungsten plated thinly with gold, have the correct weight, correct or near-correct dimensions, and are professionally produced in China.
The weight and dimensions of a coin of .999 fineness such as the Maple Leaf cannot be replicated precisely by a gold plated tungsten core, since tungsten has only 99.74% of the specific gravity of gold. However, forgeries of alloyed gold coins (such as American Gold Eagle or Krugerrand made from a crown gold alloy with 22 karats = .917 fineness) may have the correct weight and dimensions because of the lesser density of the alloy. Such forgeries can be detected by testing their acoustic, electric resistance or magnetic properties. The latter method uses the fact that gold is weakly diamagnetic and tungsten is weakly paramagnetic. The effect is weak so that testing requires strong and sensitive conditions (e.g. a gold coin hanging from 2 m long pendulum or placed on Styrofoam floating on water), but such tests can be performed without special equipment.
The rationale for biting a coin was the supposed widespread dissemination of gold-plated lead coins in the 19th century: since lead is much softer than gold, biting the coins was an effective test. While fine gold is softer than alloyed gold, and galvanized lead is still softer, biting coins can only detect the crudest of forgeries. In addition, all "gold" coins minted for circulation in the UK and America since the Tudor period (1485–1603) contained copper which made them more durable and thus hard to bite.
"This cliché might find its origin in the crude testing method used by American prospectors during the 19th century gold rush. They bit the gold nuggets they found to be sure that they were not fool's gold".
Olympic champions often pose biting their gold medals, even though the medals are no longer made of solid gold. Only at three Olympics (in 1904, 1908 and 1912) were medals made of solid gold but were also smaller. David Wallechinsky commented in 2012 that "It's become an obsession with the photographers. I think they look at it as an iconic shot, as something that you can probably sell. I don't think it's something the athletes would probably do on their own."
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