The 10% is copper and/or copper and silver. Never known it to be anything different with the possible exception of the $10 bimetallic commem. And I anit sure on that one off the top of my head.
I was reading recently where in jewelry gold is cut with aluminum, but copper sounds reasonable for coins.
Commemoratives are the same composition as the last gold coins minted for circulation, 90% gold and 10% copper and are also issued in the same weights, a $5 gold piece contains 0.241875 Toz of gold. The American gold Eagles are 91.67% gold, 5.33% copper and 3% silver and contain a full Troy ounce of gold (or even fractions thereof). The classic silver dollar and the classic double Eagle obey the classic 16:1 ratio of gold to silver value, a silver dollar contained about 0.77346 Toz of silver worth $1 a double eagle contained about .09675 Toz of gold worth $20. One dollar of gold would be 0.048375 Toz and 0.77346/0.048375 = 15.988. The Original Sixteen-to-One mine is still in operation in the foothills of California, now operated by a small group of die hard miners.
I'd imagine with jewelry they use aluminum because it is lighter and less likely to turn different colors rather than with copper. But I'm not jewelry expert nor do I wear any.
It depends on what color they want the jewelry to be. White gold for instance has the same gold content as yellow gold but they use silver for the rest of the alloy to give it the white (silverish) color. They use copper and other metals to make red gold.
The Red Book of United States Coins published by Whitman gives the composition specs for all US coins. Chris
