Gold - one of the world's most prized metals and commonly used in fine jewellery. It is most often found in jewellery as yellow or white gold when it is found in various levels of purity described as carats (or karat in the USA). 24 ct (carat) gold is pure gold and has 24 parts out of 24 entirely as gold. It is unusual to find 24ct jewellery as it is too soft to be practical. More commonly in the UK it will be found as 9ct, 18ct or 22ct. In the USA 10ct and 14ct are also common. Obviously if only 9 out of 24 parts of the item are actually gold, then the other 15 parts must be something else.
In the case of Yellow gold the remainder will usually today be silver, but older, or more "orange" gold will be alloyed with copper.
White gold on the other hand, may be an alloy of gold with either palladium, zinc or nickel. Unfortunately nickel is often the cause of dermatitis and is most often responsible for the allergic reactions which many people have to jewellery. If white gold is your weakness and you have metal allergies, you must be very careful about the source of your gold.

 What is White Gold? - There is actually no such thing as white gold, gold is yellow!
There are however gold alloys which appear white, silvery, or grey.

White gold alloys became fashionable in the 1920's, mainly as a substitute for platinum. At least three patents were issued for different "recipes" of white gold alloys during the 1920's, using different components to produce the whitening or "bleaching" effect. Simply mixing a white and a yellow metal together does not just produce a pale yellow colour, alloying produces a difference in the atomic structure which alters the reflectivity of light of different wavelengths and hence the perceived colour.
The commonest metal which causes a significant bleaching effect in gold is nickel. It has the great advantage of being inexpensive, and in 18 carat alloys, producing a good colour match for platinum. It has the serious defect that it commonly causes dermatitis, through allergic reactions when worn in contact with the skin. It is also considered to be slightly carcinogenic. There are EEC Regulations which cover the use of nickel in jewellery and all or most new jewellery sold in the EEC is now nickel-free, or at least "nickel-safe". Most American and Italian white gold alloys use nickel.
A typical nickel containing white gold alloy might be, in parts per thousand: Gold 750, Copper 55, Nickel 145, Zinc 50.

The other metal which is ideal as a constituent of white gold alloys is palladium, which is a close relative of platinum. Its main disadvantage is that it is quite expensive, and has a high melting point making it difficult to work.