Understanding GIA’s Colored Stone Grading System

The following are the steps taken by trained gemologists to evaluate the tone, hue and saturation of gemstones.

• The stone is first identified and classified according to its gem type.
• The stone is then thoroughly cleaned in order to remove any dirt, dust and grime.
• While holding the stone by its girdle against a neutrally colored background, the gemologist inspects the stone from a comfortable distance by looking at it face-up. The distance is usually about 18 inches.
• A diffused and aylight-equivalent light source is required, which is usually place about 10 inches above the stone.
• Then the gemologist rocks the gemstone back and forth (to an angle of about 30 degrees) while he makes the necessary judgments.

Measuring the tone

The illustration shows the various degrees or levels of gemstone tone. They range form very light (#2) to very dark (#8). The illustration shows that the sample gemstone has a tone that matches #5, which is a medium tone.

Measuring the hue

The adjoining illustration shows how the gemologist determines the hue of the gemstone. The gemstone in the illustration is predominantly green in color, but has a tinge of blue. The gemologist moves the gemstone around in the face-up position. The hue of the stone can be determined from the color seen in bright flashes. If one of the windows of the gemstone is so large that it occupies more than half of the face-up area, then the color exhibited in this window or facet is the dominating color, while the color visible in the brilliance flashes is an additional color.

Measuring the saturation level

As the final step, the gemologist determines the saturation level of the gemstone color. The gemstone is considered to have a vivid saturation if there is no incidence of a grayish or brownish appearance.

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