A troy ounce is a measurement used to weigh precious metals, such as gold, as well as gemstones. The troy ounce is equal to 31.1034768 grams. There are 14.58 troy ounces in a pound, or 12 troy ounces in a troy pound.
The troy ounce dates back to the Middle Ages, when it was likely used originally in Troyes, France. The weight was first used in England around the year 1400, and was adopted officially as the British Imperial troy ounce in 1824. In the United States, it was determined to be an official weight of measurement for coinage by an act of Congress on May 29, 1828. It is the only unit of the troy measurement system still in use, as of 2015.
Troy weights differed slightly by location, before being standardized by the British in 1527. There were, for example, Paris troy, Bremen troy and Holland troy. Because of the various weights, the value of each troy could differ by several percentage points.
Both the British and American troy systems, which are equal in weights, are based on the grain. There are 480 grains to each troy ounce, compared to the 437.5 grains per non-metric ounce used by the more common avoirdupois system. Because of this discrepancy between the two systems, even though a troy ounce is heavier than an avoirdupois ounce, a troy pound is actually lighter.