The gram is the most widespread unit of weight for precious metal jewelry. However, the United States and England also employ a system of weights called the “troy weight.” In this system, a pound of gold or silver equals 12 ounces, and 1 ounce equals 20 pennyweights.
It is important to distinguish troy weights from avoirdupois weights. Avoirdupois weights are used in the United States for almost everything except precious metals and gems. In contrast to a troy pound, which equals 12 ounces, one avoirdupois pound equals 16 ounces.
The “troy” may have been named after a weight used at an annual fair held in Troyes, France during the Middle Ages.
A pennyweight is a unit of weight equaling 1/20th of a troy ounce. In the Middle Ages, it was the weight of a silver penny in Britain. The pennyweight is used mainly in the American jewelry trade.
1 pennyweight (dwt) = 1.555 g = 0.05 oz t
1 troy ounce (oz t) = 31.103 g = 20 dwt
1 gram (g) = 0.032 oz t = 0.643 dwt
Other Weights and Measures:
A grain is a unit of weight equaling 1/24th of a pennyweight. A troy ounce is equal to 480 grains. This was one of the earliest units of weight for gold. It was originally the equivalent of one grain of wheat.
The tael is a Chinese gold weight. One tael = 1.2034 ounces troy of .999 fineness.
A carat can be a unit of mass (weight) or purity. When describing a unit of mass in gemstones, it equals 0.2 gram or 200 milligrams. The carat, abbreviated as “ct,” is also used to describe the purity of gold and platinum alloys . In the United States and Canada the preferred spelling is “karat” to distinguish from “carat” the unit of mass. One karat is 1/24 pure, so 24 karat gold is pure gold. Both karat and carat derive from the Greek word for the seeds of a carob tree. In ancient times, carob seeds were used as counterweights when weighing gems and jewelry because they had a reputation for having a uniform weight.
Historically, a “fine ounce” is a troy ounce of 99.5% pure gold and a “standard ounce” is a troy ounce of 91.66% (22K) gold.
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