The 22 kt crown gold standard, but with over 6% silver
Quarter eagles were issued for circulation by the United States Mint from 1796 until 1929; half eagles from 1795 until 1929; eagles from 1795 to 1933; and double eagles from 1850 to 1933, although for each of these ranges of years there were occasional gaps in production. The diameter of quarter eagles was 17 mm; of half eagles 21 mm; of eagles 27 mm; and of double eagles 34 mm.
Originally the purity of all circulating gold coins in the United States was the traditional English crown gold standard of 22 karats (11 parts gold to 1 part alloy). The weight of quarter eagles was 67.5 troy grains (4.37 g); of half eagles 135 troy grains (8.75 g); of eagles 270 troy grains (17.5 g). This resulted in a gold content of 0.516 troy ounces for the eagle.
Gold content drops to a low of 21.58 kt or 89.92% in 1834
In 1834, the mint value of gold to silver of 15:1 (6.11% silver) was changed to 16:1 (5.73% silver) and the metal weight-content standards for both gold and silver coins changed, because at the old ratio and content, it was profitable to export and melt U.S gold coins. Also, the gold proportion was dropped from 22 karats (.9167 fine) to 21.58 kt (.8992 fine).
The enduring 90% gold 10% copper standard of 1837 for circulating US coins
In 1837 a small change in the fineness of the gold (increased to exactly .900 fine) was made, and all silver was completely removed (this was in keeping with English tradition, where gold sovereigns then (and now) contained no silver). The new 1837 standard for the eagle was 258 troy grains (16.718 g) of .900 fine gold (with the alloy remainder for all U.S. coins after 1837, to .100 copper and no silver), with other coins proportionately sized. The 1837 standard resulted in a gold content of only 0.9675 troy ounces of gold per double eagle and 0.48375 troy ounces for the eagle. It would be used for all circulating gold coins until U.S. gold coin circulation was halted in 1933.
Post-1982 eagle modern commemorative coins
As part of its Modern United States commemorative coins program the United States mint has issued several commemorative eagle coins. In 1984 an eagle was issued to commemorate the olympics, and in 2003 to commemorate the Wright Brothers first flight at Kitty Hawk. The pre-1933 .900 fine gold standard was restored, this would also be used in half-eagle gold commemoratives as well. The coins would be identical in fineness and size of their pre-1933 counterparts of the same face value. In 2000 a unique eagle the 2000 Library of Congress bimetallic ten dollar coin was issued commemorating the Library of Congress consisting of equal weights of an approximately 1/4 oz .9995 fine platinum core and a .900 fine gold outer ring.
Article Source: en.wikipedia.org