The fineness of a precious metal refers to the ratio of the primary metal to any additives or impurities traditionally expressed as parts per 1,000. A piece of alloy metal containing a precious metal may have the weight of its precious component referred to as its fine weight. For example, 1 troy ounce of 18 karat gold (which is 75% gold) may be said to have a fine weight of 0.75 troy ounces. Many precious metals are used in the form of alloys.