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Colored Stone Grading - Color

Color Name

Our color grade names the overall color saturation, tone, and hue of the gem, in that order - Vivid Medium Orange-Red, for example. We grade each component of the color name, basing our grades on the descriptive system for color developed by GIA.

Saturation

Numbers 1—6 represent a color's strength, intensity, or saturation. Another commonly-used term is purity.

1 - Brownish or grayish colors
2 - Slightly brownish or grayish colors
3 - Very slightly brownish or grayish colors
4 - Moderately strong colors (no trace of brown or gray)
5 - Strong colors
6 - Vivid colors

Tone

Numbers 0—10 describe the brightness of the color. In practice, only levels 2 through 8 apply to transparent colored stones — tones lighter or darker are nearly impossible for the human eye to detect. The more valuable gems tend to fall within medium light to medium dark tones and higher saturations.

0-1 - Colorless or white
2 - Very light
3 - Light
4 - Medium light
5 - Medium
6 - Medium dark
7 - Dark
8 - Very dark
9-10 - Black

Hue

Hue describes the color names that show their position on the color wheel. Note that the dominant color is capitalized and when both are capitalized and separated by a dash, each color has equal weight. The hues we use are as follows:

Important Notes on Hue: The color descriptions in the GIA system are loosely based on the Munsell color system with modifications based on natural distributions of hue that are found in gems. Other color systems are perhaps more precise, but Munsell’s basic idea that the descriptions should match the human perception of color has a solid experimental foundation and many practical applications in today's world.

The pink hues are not listed in the GIA descriptive system. We have chosen to use them because of trade practices. Fancy sapphires such as Padparadscha are usually described in terms of pink and orange, even on GIA Identification reports. Certain hues of tourmaline, morganite, and diamond are also named pink. Fancy pink sapphires often have overtones of purple, so we've included purplish Pink and similar hues.

The GIA color description system uses brown and gray primarily as modifiers or masks of color saturation. In nature however, we see many gems that can best be described as brown or gray being the dominant color, such as yellowish Brown, orangey Brown, bluish Gray, etc. Brown colors can show different levels of richness or saturation with overtones of yellow, orange, or red. Gray colors have overtones from the cool side of the spectrum. We use such hue descriptions where appropriate.

Certain portions of the color wheel are more precisely divided into distinct hues - the yellow to green range, the green to blue range, and the variations of purple and violet. This is because in the natural distribution of gems, we see many subtle variations of hue within these ranges. Within the gem trade, for such gems as Aquamarine, values can vary significantly based on subtle distinctions of hue.

Lighting Effects

The interaction of light with the color of gems and the human eye is very complex. Changes in lighting will cause changes in the perceived color of gemstones. One primary factor is color temperature. The Kelvin scale is often used in the measure of the color temperature of light sources. The traditional standard by which gems are graded is north daylight at noon (5500-6000Kelvin), which is a balanced or white light. In the gem trade, fluorescent lights with this color temperature are used for both diamond and colored stone color grading.

Different types of light sources produce light with different color temperatures. An incandescent lamp produces light at a different color temperature that does a fluorescent lamp, and different fluorescent lamps are rated at different Kelvin.

A light source such as the sun produces light at different color temperatures at different times of the day. Filtering agents in the atmosphere, such as pollution or fog, can affect color temperature and color perception. Natural light in the tropics is very different than that in the temperate and arctic zones. This all means that you must be aware of the light source when you view a gemstone. A good rule of thumb is to view a gem in different types of light whenever possible, or to view it next to a comparison stone with which you are familiar.

Some gems like sapphire, ruby and emerald may lose color value when the lighting environment is changed from daylight to incandescent light. Incandescent light will make red stones like ruby look better, but will make sapphires look worse. This effect is known as "bleeding," and literally causes the gem to lose some of its tone and saturation. Other gems such as tourmaline and garnet may exhibit a gray or brown mask in incandescent lighting, or the primary or secondary hues may change.  Many gems appear to darken when they are set into jewelry, sometimes with beneficial effect, sometimes with negative consequences. Some gems, like tourmaline, almost always change to some degree.

To some extent, strongly pleochroic gems are more prone to changes in appearance, especially since different colors are seen along different axes of the stone. In most cases this is a fault, the degree of which depends upon the effect on the gem's beauty. Good cutting can minimize these effects.

A very fine gem either holds its color or holds its beauty when the lighting environment changes. If changes occur in color that noticeably or negatively affect beauty, it is considered in the overall color grade and will be noted in the Lighting Effects.

Overall Color Grade

We color grade by combining the three components (saturation, tone and hue) with the lighting effects into a color-quality score of 1 - 10.  This grade will reflect actual quality of color, plus the desirability and rarity of the color of the stone. The overall color grade tends to follow what color scientists call "color gamut limits," where each hue reaches its optimum saturation at a certain defined tone.

The gamut limits vary for each hue. Orange is light and bright, red is dark and rich. A yellow stone may have a tonal value of 3 but a color grade of 10, if the saturation is high and the hue pure and pleasant. Absent gray or brown, yellows reach their brightest and richest saturation around 20-30% tone, oranges around 20-40%, greens around 60-75%, reds at 70-80%, and blues and purples at 75-85%.

The gem market has known this fact for centuries — the most desirable and valuable colored gems closely follow these limits. Thus, all other factors being equal, a red stone such as ruby would be perceived at its finest color at 70 - 80% tone. Pure hues are favored over mixed hues, but are very rare in nature. The presence and percentage of secondary hues and/or masks will naturally affect the grade. Rare and unusual colors may receive a higher grade if the color is perceived as desirable.