Purity refers to the amount (by weight) of pure gold or silver an object
has. The most common way of describing purity is by Carats
(Karat in the US). Carats are just the purity expressed in units of 24, such
that 100% equals 24 carat and 50% equals 12 carat.
Carats are used primarily in the jewellery industry. For bullion, the
industry uses either percentages or fineness (fine being another word for
pure). The table below shows the most common purities.
Percentage
|
Fineness
|
Carat
|
99.99%
|
9999
|
24
|
99.50%
|
9950
|
-
|
99.90%
|
9990
|
-
|
91.66%
|
9166
|
22
|
75.00%
|
750 on jewellery
|
18
|
58.33%
|
585 on jewellery
|
14
|
In the wholesale investment precious metal markets, physical gold is
traded in bars of approximately 400oz with a purity of 99.5% or more and
these are often referred to generically as “nine nine five” bars. For silver,
the market trades in 1000oz bars of 99.9% purity, also called “three nines”
bars. The reason wholesale gold is traded at 99.5% is because it is much
cheaper to refine to this purity than 99.99% (or “four nines”), which
requires additional processing (see here for the refining process).
In retail markets, customers usually prefer coins and small bars with a
purity of 99.99%, notwithstanding the additional cost. However, the 22 carat
American Eagle and South African Krugerrand gold coins are very popular.
Silver is usually only sold as 99.9%, although “four nines” silver coins
are available. Investment platinum is mostly sold at a purity of 99.95%.
Purities greater than 99.99% are generally only useful in certain
industrial/scientific equipment and processes where the small impurities
control the electrical and mechanical properties of the metals.
There are international standards on determining the purity of a product.
For example, Australian Standard 3515.4-2007 says that for something to be called 99.99%
the minimum purity has to be 99.9875%.