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 comes from the Greek word for 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 grams), so there are 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 value 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.
However, in the real world, pricing is not this simple, even for diamonds. Other qualities such as color or clarity affect the price of the stone. With colored gemstones, this carat law is even more muddled and unrealistic. For example, a high quality ruby of three carats may be more valuable and rarer than a ruby of over 200 carats, as huge rubies are common, but full of serious inclusions. These giant specimens are impossible to cut into fine stones because of their flaws; therefore, their value is very low.
Also, remember that size and weight are different. Size refers to the physical space or volume that a stone occupies, while weight refers to the gravitational pull of the object. For example, one cubic millimeter of sapphire will be twice as heavy as one cubic millimeter of opal because the sapphire's specific gravity is twice that of opal. Please see our page on calibrated sizes for more information.
- First Published: September-21-2006
- Last Updated: May-14-2014
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