Fineness refers to the weight of fine metal such as silver or gold in a precious metal object such as a bar, a coin or a piece of jewelry.
It is calculated as a proportion of the total weight which may include base metal alloys or impurities.
Alloy metals such as nickel and copper are added to increase hardness and durability, to reduce cost per weight or to change color.
Fineness can be expressed as the quantity of fine metal in parts per thousand.
For example, sterling silver is 0.925 pure since it contains 92.5% silver. The remaining 7.5% is normally copper.
Karat as an expression of fineness is reserved for gold and is based on parts per 24.
For example, 18k gold is 18 parts gold out of 24 or 75%. 24k gold is considered 100% pure. Although it is impossible to achieve a fineness of 100% gold, the Royal Canadian Mint regularly issues coins that are 0.99999 pure and the Perth Mint produced a 0.99999999 pure coin in 1957.