Weight is an important quality of gemstone value. Often the larger the stone, the more valuable it is, as larger stones are less common. However, it is important to remember that qualities such as color and saturation are also important in determining a stone's value.
The English word carat come from the Greek keration ('little horn'), referring to the shape of the seedpods of the carob tree. These seeds are small and uniform in weight; therefore, they were a standard for weighing gemstones in early times.
Around 1920, the metric carat was adopted as an international standard. One carat equals a fifth of a gram (0.20 gram), or five carats to the gram.
Generally, as a gemstone's carat weight increases, so does the price per carat. A fun way to look at stone values is the following: since the late 14th century there has been a law for diamonds that is known as the 'Indian Law' or 'Tavernier's Law':
Wt² x C = Price per Stone
The following shows how the price of a diamond might increase with this formula applied to a $1000 a carat base price.