One of the most commonly used weights in precious metal circles, the troy ounce is equivalent to 31.1034768 grams. The measure originated in the Middle Ages when it was used by precious metal traders in Troyes, France. The ounce is the only unit in the troy system that's still widely used today and it's used to set the price of metals like gold, silver and palladium. If the spot price of gold is £400 per ounce, that always means troy ounces not the commonly used avoirdupois ounce. The 1oz bullion coin is one of the most popular options for collectors - each contains a troy ounce of precious metal and you can browse our entire range at coininvest.com.