The gold eagle coin was a base-unit of denomination issued only for gold coins by the United States Mint. The eagle was the largest of the four main decimal base-units of denomination used for circulating coinage in the United States prior to 1933, the year when gold was withdrawn from circulation. These four main base-units of denomination were the cent, the dime, the dollar, and the eagle, where a dime is 10 cents, a dollar is 10 dimes, and an eagle is 10 dollars. Just as the cent base-unit of denomination served as the basis of the copper half cent, the copper cent, and the copper double-cent coins and just as the dime base-unit of denomination served as the basis of the silver half-dime, the silver dime, and the silver double-dime coins and just as the dollar base-unit of denomination served as the basis of the silver quarter-dollar, the silver half-dollar, the silver dollar, and the gold dollar coins, the eagle base-unit of denomination served as the basis of the gold quarter-eagle, the gold half-eagle, the gold eagle, and the gold double-eagle coins.
With the exceptions of the gold dollar coin, the gold three-dollar coin, the three-cent nickel, and the five-cent nickel, the unit of denomination of coinage prior to 1933 was conceptually linked to the precious or semi-precious metal that comprised a majority of the alloy used in that coin. In this regard the United States followed long-standing British and European practice of different base-unit denominations for different precious and semi-precious metals. In the United States, the cent was the base-unit of denomination in copper. The dime and dollar were the base-units of denomination in silver. The eagle was the base-unit of denomination in gold.
During the 1850s following the discovery of gold in California, an additional fifth base-unit of denomination was proposed, called a "union" whose value would have been 10 eagles or, equivalently, 100 dollars. Although prototypes of union and half-union gold coins were minted and presented for inspection to Congress, Congress did not pass a law that authorized the minting of union-denominated and half-union-denominated coins for circulation because the double-eagle coin was viewed as a substantial sum of money for the general public at the time and a half-union or union coin would primarily have circulated only among banks and other financial institutions. Despite this absence of a half-union coin for general circulation, a single issue of a gold commemorative $50 gold coin was minted in 1915. All of these gold coins have been retired and are no longer produced for circulation. Although technically still legal tender, the value of the gold content vastly exceeds the face value in the years following 1971, the year at which the value of the dollar ceased being pegged to a fixed amount of gold. They are generally held either by coin collectors or as an investment in gold.
Gold quarter eagles were issued for circulation by the United States Mint from 1796 until 1929; half eagles from 1795 until 1929; gold eagles from 1795 to 1933; and gold double eagles from 1850 to 1933, although for each of these ranges of years there were occasional gaps in production. The diameter of gold quarter eagles was 17 mm; of gold half eagles 21 mm; of gold eagles 27 mm; and of gold double eagles 34 mm. The purity of all circulating gold coins in the United States was 22 karats (11 parts gold to 1 part alloy). The weight of gold quarter eagles was 67.5 troy grains (4.37 g); of gold half eagles 135 troy grains (8.75 g); of gold eagles 270 troy grains (17.5 g); of gold double eagles 540 troy grains (1.125 troy ounces or 35 g).
The United States' circulating eagle denomination from the late 18th century to first third of the 20th century should not be confused with the American Eagle bullion coins which are manufactured from silver (since 1986), gold (since 1987) or platinum (since 1997). Bullion coins carry a nominal face value, but their value is mainly dictated by their troy weight and the current precious metal price.
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