Most of the people think that only diamonds have grading but there is also a set of criteria to determine the grade of the gemstones.
We have evaluated and summarized the following for grading:
A gem’s value is primarily based on its color. Here are the basics you need:
Color of gems
There are three primary colors (red, blue, yellow) and three secondary colors, which are the result of mixing primary colors (purple, orange, green).
Nature often displays tertiary colors, such as red-orange, yellow-green, or blue-violet, which are a primary color mixed with a secondary color.
When a color is mixed with gray, white, or black, we need the (overlapping) definitions of saturation, hue and tints.
Saturation
Saturation expresses the attribute of perception of gray of the same lightness. All grays have zero saturation. Theoretically a 100% saturation means there is 0% gray in a color.
Hue
Hue is color perceived to be red, purple, yellow, green etc., meaning white, black and gray have no hue.
Tint
A color mixed with white is a tint. A tint is lighter and less saturated than a color without the addition of the white.
Color Grade and Tone
Color grade describes the strength of the main color compared to other colors visible in the stone.
A 100% color grade in blue for example would imply that there are no other colors (like purple or violet) visible in the stone.
Color Grade
Excellent: 1-2
Very Good: 3-4
Good: 5-6
Fair: 7-8
Poor: 9-10
The color grade is simply based upon the purity of primary color. For example, the greener the emerald, the redder the ruby, or the blue sapphire, the lower the numerical grade on an AGL scale. The reason the 3.04 sapphire is a 3.5 in this sample is the fact the stone has 65% blue as its primary color. It is that simple. The less primary color, the higher the numerical color grade. For example, a 60% primary color would probably be graded a 4 color.
The color scan will generally break down into three perceived colors that total 100%. In the sample, the sapphire is 65% blue, 20% violet and 15% green.
If there was also no gray in the stone, we would have a 100% saturation with a 100% color grade, but such a stone has never been found.
Generally the rule applies: the purer the color the higher the grade. But grade is nothing without tone:
Color Tone
Any color grade has to be seen in combination with tone.
In the extremes a colored stone could be white (light 5) or black (dark 95) with just a hint of color.
Tone Scale
Very Light: 0-20
Light: 25-35
Light-Medium: 40-50
Medium: 55-65
Medium-Dark: 70-85
Very Dark: 90-100
Basically, tone describes the lightness or darkness of a gemstone. Color considered alone without taking into account the tone will lead to errors in interpretation. If a stone is too light in tone, the color will not be rich enough. Conversely, if a stone is too dark, it will sacrifice transparency and brilliancy.