Gold quality is determined by karat content or fineness, and each karat is 1/24th part of the entire substance.  Pure gold is 24 karats, and as it is alloyed with other metals, the gold content decreases. For example, 18k gold is 18 parts pure gold and 6 parts of alloy metals; 14k gold is 14 parts pure gold and 10 parts other metals.

 

Red or pink gold (also called rose gold) contains large percentages of copper and a small amount of silver in addition to gold.  Green gold is usually gold alloyed with additional silver or with silver that has a small percentage of copper.  White gold is primarily 75% gold with the balance nickel, or both nickel and zinc. Gold alloyed with silver alone minus nickel or zinc will be less hard than a nickel and zinc alloy and also a lighter yellow color.

 

Copper alone added to gold makes the pure gold harder and more difficult to work; therefore, some percentages of copper and silver are usually combined in the alloy. White gold- formerly made of an alloy consisting of gold, silver, and/or palladium (from the platinum family) is now also alloyed with nickel, copper, and zinc to produce more ideal melting characteristics.