Most jewellery comes with the gold that is mixed with other metals, such as silver, copper, even platinum and palladium and rhodium (in the case of white gold).
The gold purity is expressed as follows:
- 24 karat is 999.99 fine
- 22 karat is 916.6 fine
- 18 karat is 750 fine
- 14 karat is 585 fine
- 9 karat is 375 fine
24 carat is pure gold with nothing added. This is the purest gold available. Also has a fineness of 1000 but this is expressed as 999 being 999 parts per 1000. This is because it is very difficult to get real pure gold with absolutely no impurities and possible is for legal reasons also. This applies to gold coins a lot. Canadian maples, for example are listed as 99.999% purse gold.
18 carat gold is more popular for gold jewellery with a 75 percent gold and 25 percent other metals ratio, usually silver or copper or a mixture of both.
The 14 carat standard is used more extensively in industry and for such things as pen nibs, circuit boards etc.
It is also used in such jewellery as bracelets where more durability is required due to more use.
There is also a 10 carat, containing 41.7 percent gold and known therefore as 417. This is really just a cheaper version of the 14 carat and used for cheaper jewellery.
Several different metallic elements are alloyed with Gold, and some are used specifically to produce a certain colour or tone in the Gold.
The main metals alloyed with Gold are copper, silver, palladium, nickel, zinc, and iron. White Gold, which has become very popular in jewellery, is mainly alloyed with nickel and zinc, and occasionally palladium. White Gold resembles the colour of Silver, but it is far more resistant to corrosion and will not tarnish like Silver. Rose Gold, which has a slightly reddish tone, is alloyed mostly with copper. Green gold, which appears greenish-yellow, is alloyed with silver, and Blue Gold, which is gold with a whitish-blue tone, is alloyed with iron.
Some gold-coloured jewellery is made out of cheaper metals and coated with a very thin layer of Gold. Such jewellery is known as "Gold-plated". Through normal wear, the layer of gold can erode if it is too thin, exposing the underlying metal.
Gold is one of the heaviest substances on earth. When pure, it has a specific gravity of 19.3. The international weight measurement of Gold is the troy ounce. One troy ounce of gold equals 31.1 grams.