Often the source of much confusion, the difference between a karat and carat is fairly easy to understand, but may be somewhat hard to remember. A karat is simply a unit of measurement to indicate the purity of a metal, usually gold. The term carat is used to note the weight of a precious stone, especially diamonds.
The term karat is necessary, especially for gold, simply because the nature of the metal means it must be made into an alloy. Gold is a very soft metal that is not able to used practically as jewelry in its purest form. It would quickly become deformed and scratched. Therefore, jewelry makers must do something to fortify the metal. This task is accomplished by mixing gold with a harder metal.
The karat is responsible, in most cases, for determining the relative value of the gold piece. The difference in price between 10k, the lowest purity that gold can be and still be considered gold, and 18k, the highest purity of gold jewelry commonly available, can be significant. Gold designated as 10 karats is less than half gold, while eighteen karat gold is 75 percent gold. One that's twelve karats is half gold and half another metal or metals. While it is available for sale, it is not one of the most common purities.
The term carat applies to weight and is based on the metric system. One carat is 200 milligrams (seven-thousandths of an ounce). In other words, a precious stone weighing one gram (35-thousandths of an ounce) would be five carats. If the term is used correctly, any time one sees the word carat, it will apply to the weight of a precious stone, especially diamonds. If the weight is below one carat, the measurement is then often referred to as a fraction, usually a quarter, half or eighth of a carat.
While the karat is responsible for much of the worth of gold, the carat may be only one of the major factors reflecting the value of a precious stone, especially diamonds. The diamond quality is determined by the four C's: cut, clarity, carat, and color. The most valued diamonds are those that have good clarity and are colorless, but size also makes a difference.
It should also be noted while there are few diamonds weighing more than a couple of carats, other precious stones are commonly found weighing a dozen carats or more. Therefore, while size does matter and does help determine value, the type of stone is a critical factor. However, even in jewelry, bigger is not always better. Many precious stones are often cut down in size so that they can be placed in conventional jewelry.