The world’s most important unit of weight for precious metals trading is the ounce, or more precisely the troy ounce (also known as the apothecary ounce), which has a weight of 31.1034768 grams. The ounce as a unit of weight was already in use in ancient Rome. Its weight is one twelfth (lat. Unica) of a Roman pound. In the USA, the ounce is still a common weight measure today. All prices for silver and gold are calculated in ounces, the price for gold in most cases in US dollars.