What is the troy once?

A troy ounce is a unit of measure used primarily for weighing precious metals like gold, silver, and platinum. One troy ounce is equivalent to approximately 31.1035 grams. The troy ounce differs from the more commonly used avoirdupois ounce, which is about 28.35 grams. The troy system has been in use since the 14th century and is named after the city of Troyes in France.