The troy ounce is a unit of measurement commonly used to measure the weights of precious metals, namely silver, gold and platinum. Troy ounces were once used for all sorts of measurements but are today limited to the weighing of these materials.
Often said to originate in a marketplace in Troyes, France, the troy ounce likely derived from elements of the Roman measurement system. A troy ounce is about 10 percent heavier than the avoirdupois ounce, which is used to measure most other materials. The weight of a troy ounce has been standardized at approximately 31.10 grams by international convention.