Google

Gold As Prized for its beauty and versatility

Prized for its beauty and versatility, gold is the classic setting for most jewelry. It’s also the most malleable of all metals — so soft it can’t be used for jewelry in its purest form.
The standard measurement of gold is the karat. Pure gold is 24 karats, meaning 24 out of 24 parts are gold. To increase its strength, gold is combined with other metal alloys. For example, 18K gold is 18 parts gold and 6 parts other alloys; 14K gold is 14 parts gold and 10 parts other alloys. 10K gold is more durable, with 10 parts gold to 14 parts other alloys.
Alloys used with yellow gold include copper and silver. Rose gold is created by combining gold with large amounts of copper, while green gold results from mixing gold with copper, silver and zinc. When creating white gold, pure gold is combined with copper, zinc and nickel or palladium.
Since antiquity, yellow has been the color most associated with gold. White gold is a beautiful complement to exceptionally white and bright diamonds. Also, white gold jewelry is plated with rhodium, a shiny metal that increases the whiteness and strength of gold. Sometimes, white gold is confused with platinum, though they are entirely different metals. White gold and platinum vary in strength, resistance to scratches and shades of white.
In some jewelry pieces, white and yellow gold are paired together, producing a beautiful two-tone look. You can also find yellow, white and rose gold pieces or white and rose.
Resources
http://www.jewelrywise.com/jewelry-101/article/gold