Like diamonds, colored stone grading also consists of the 4 Cs. However, colored stone grading is even more complex than diamond grading. Therefore, I will keep things simple and give you a boiled down version of how this grading system works.
To begin with we will only be concerned with transparent gemstones.
The 4 Cs consists of color, clarity, cut and carat weight. The combination of the 4 Cs
is what determines the quality and hence the rarity of each gemstone which in turn determines the price you pay.
By far, color is the most important of the 4 Cs. This is broken down into three components: hue, tone and saturation. The combination of these components determine the color grade.
So what are you looking for? A gemstone that has a nice even color that is not so light that it appears pale nor too dark that it is opaque. And finally, the color should be strong and evenly distributed throughout the entire stone.
Refers to the internal characteristics called inclusions and surface blemishes. Transparent gemstones are broken down into three categories or types: Type I - usually clean or free of inclusions, Type II - usually included and Type III - almost always included. (Now you can begin to see why this is such a complex system, but this is more than you need to know because in the end your eyes will be the judge.)
Refers to many things which we summarize as the make of the gemstone. This includes both the proportion and finish. It also includes the shape as well as the face up outline and facet design.
Carat weight refers to the actual weight of the gemstone which is weighed on a
gemstone scale. It is the most clear-cut value factor. As with diamonds, don't confuse carat weight with the size of the gemstone. Two gemstones can have the same carat weight but look
dramatically different in size based on their proportions as well as their density. As stated above, this is due to the "cut" of the gemstone. Colored gemstones are often referred to by millimeter measurements, such as 10.00 x 8.00 mm, rather than weight.
Carat
weight is expressed as follows: (Just like there are 100 pennies in a
dollar, there are 100 points per carat.) So one point, expressed as
0.01carat equals 1/100th of a carat.