High Quality Content by WIKIPEDIA articles! Troy weight is a system of units of mass customarily used for precious metals, black powder, and gemstones. Named after Troyes, France, the troy system of weights was known to exist in medieval times, at the celebrated fair at Troyes in North Eastern France. One cubic inch of distilled water, at 62 °F (17 °C), and at a barometric pressure of 30 inches of mercury, was determined to weigh 252.458 troy grains (gr). There are 12 troy ounces per troy pound, rather than 16 avoirdupois ounces (oz) in the avoirdupois pound (lb) as in the more common avoirdupois system. The avoirdupois pound is 147 12 ( 14.583) troy ounces. Although Troy Ounces are still used to weigh gold, silver and gemstones, the Troy Pound is no longer used.