Caratage is a measurement of gold content. The stamp of "18 carats" on a piece of jewelry indicates it is not pure gold, but a percentage of gold mixed with other metals. Carats are alternatively described or abbreviated as karat, kt., ct., or fineness. Different countries have different standards for measuring and selling gold in carats.

The price of a gold object depends upon the gold carat, or how much pure gold is in the object. Pure gold is described as 24 carats; 18 carat gold contains 75 percent pure gold, or is 18/24ths gold. When described in terms of "fineness," 18 carat gold is assigned the descriptor of 750.

Different countries have different regulations in terms of selling and classifying gold and gold carat. Some countries only allow certain carats be sold. In the United Kingdom, for example, nothing below 9 carats can be marketed as gold.

Tolerance levels are also a factor in classifying gold. Tolerance levels describe the amount of impurities allowed. In the United States, 3 parts per thousand are allowed to be impure. That means gold that is 74.97 percent pure can be marketed in the United States as 18 carat.

Using 18 carat gold has many advantages over using pure gold. It's cheaper, for one. It also allows for different colors, as other alloy metals can be mixed with pure gold to provide variations. Impure gold is also stronger and harder, making it less likely to show signs of wear, scratching, distortion and other damage. On the Vickers scale, which measures metals' hardness, 18 carat gold is 130 while 9 carat gold is 120. This is moderate; sterling silver, for instance, measures 90, while platinum measures 165 and titanium measures 300.

When alloy metals are mixed with pure gold, it can change the resulting mixture's color. Green, pale yellow, yellow, rose/pink, red and white are all possibilities with 18 carat gold, depending on which alloys are used at what percentage. Yellow gold is mixed with metals like copper and zinc; rose gold is mixed with copper; and white gold is mixed with silver and palladium, covered with a rhodium plating. White gold is usually more expensive than yellow or rose gold, which are similar in price to each other.

Suitable for all kinds of jewelry, from finely crafted items to everyday wear, 18 carat gold is one of the most versatile choices of metal. In Southern Europe and the Mediterranean, it is the most common gold found in jewelry. In the United States and Northern Europe, 18 carat gold is one of the most common, with other common varieties ranging from 8 to 18 carats. However, in the United States, 18 carat gold is the most popular metal for wedding and engagement rings. In fact, it is preferred over higher grades of gold for men's wedding rings, in particular, due to its durability and pricing.