Why does the 1 oz American Gold Eagle Coin weigh more than one troy ounce?

American Gold Eagles are 91.67% gold. To make the coin more durable, the other 8.33% is composed of silver and copper. When considering the weight of American Gold Eagles, one must account for the additional alloys. The 1 oz American Gold Eagle is composed of one troy ounce (approximately 31.1 grams) of gold. The silver and copper add approximately 2.8 grams, bringing the total weight of the 1 oz American Gold Eagle Coin to approximately 33.9 grams.