Fineness refers to the weight of fine metal such as silver or gold in a precious metal object such as a bar, coin or item of jewellery.
It's calculated in proportion to the total weight which might include base metal alloys or impurities.
Alloy metals like nickel and copper are added to increase the hardness and overall durability of coins, to reduce the cost per weight or to change the colour.
Fineness can be expressed in various ways. 'Millesimal Fineness' indicates the amount of fine metal as parts per thousand.
For example, sterling silver is .925 pure since it contains 92.5% silver. The other 7.5% is normally copper.
The 'Karat' or 'Carat' method of expressing fineness is reserved for gold and is based on parts per 24.
Therefore 18 carat gold is 18 out of 24 or 75% and 24 carat gold is considered 100% pure. Although it's impossible to reach 100% gold fineness, the Royal Canadian Mint regularly issues coins that are .99999 pure and the Perth Mint first produced a .999999 coin in 1957.