Do you know that the standard measurement for precious metals is actually the Troy Ounce rather than the standard (Avoirdupois) Ounce used in pretty much everything else? 

1 Troy Ounce = 31.1034768 grams exactly
1 Avoirdupois Ounce = 28.3495g
1 kilogram = 32.1507466 Troy Ounces (pretty close but not exact) 

So where did this Troy Ounce come from? 

It came from the French town of Troyes in the Middle Ages. A standard of measurement was needed for traveling merchants to do business with the French traders. 

King Henry II adopted this system in the 12th century where it remained till the 16th century. Then in 1527, England ruled that gold and silver were to be measured in the Troy Pound (12 Troy Ounces) as compared to the commonly used Towre Pound. 

The United States only adopted the Troy Ounce for gold and silver measurement in the year 1828.