Gold has the longest and most storied history of all precious metals. Soft enough to be worked into interesting shapes, its warm color and scarcity gave it great value in early civilizations. It has been the foundation of many monetary systems, and remains important to our economy, even today.
In the United States, the karat mark has become the standard for gold purity in a piece of jewelry. Only jewelry with enough gold to merit a karat stamp can be considered real gold. Measuring karats in gold is based on a scale of 24, with 100% equaling 24 karats. 24k gold is not often used in jewelry making because it is very soft. 18k gold is 75% pure gold. Most gold will have alloys (other metals, usually copper or silver) added to it to strengthen and harden the metal. The minimum standard in the United States is 10k.
As the karat weight drops, the metal becomes more durable but less yellow. Sometimes gold that is a lower karat weight will be plated in high-karat gold to enhance the color. This is perfectly acceptable as long as you pay a fair price. Also keep in mind that gold plating will wear off with time and your jewelry may need to be re-plated.
Alloying gold with other metals creates different color and hue variations. One important item to remember is that purity and fineness does not fluctuate as a result of the color other than 24k. A ring that is 18k gold is 18k pure whether it is yellow gold, rose gold or white gold. The following are how different alloys affect the color of gold:
If gold is combined with:
Copper & Zinc, or Copper & Silver
Silver, Copper & Zinc
Nickel, Zinc, Copper & Manganese
The color will be:
Pink or "rose" gold
When buying gold jewelry, look for a stamp with a karat mark, the manufacturer's registered trademark and the country of origin.