https://en.wikipedia.org/wiki/Eagle_(United_States_coin)
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. ... Between 1838 and 1840, the silver content was reduced to zero—the eagle in 1838, half eagle in 1839, and quarter eagle in 1840, —resulting in U.S. gold coins being 90% gold and 10% copper.