Differences Between $20 St Gaudens And $50 Am Gold Eagle.

As you probably know, the double eagle was made to contain $20 worth of gold. The "spot price" at the time was $20.67 per troy ounce which meant the coin contained 0.9675 ounces. After addition of 10% copper to produce the 90% gold alloy needed for US gold coinage the final weight was 33.436 grams or about 1.075 ounces. I suspect the dimensions were adjusted to ensure the coin could be properly struck using the available coining equipment and the 34-millimeter diameter was the result.

The gold eagle is composed of 91.67% gold alloy with 1.000 troy ounce gold content. Obviously, mint officials had to choose dimensions that allow this quantity of gold alloy to be reliably struck and the eagle ended up at 32.7 mm. Other 1-ounce bullion coins like the Maple Leaf, Nugget, Onza, Panda, Krugerrand, and Philharmonic have various diameters from 30 to 37 mm. The 2009 Ultra-High Relief 1-ounce coin is extra-thick and has a 27 mm diameter. My guess is that diameters were adjusted as needed to optimize the minting process for each design.