A gemologist evaluates a gemstone’s hue, tone and saturation using the GIA Type Colored Stone Grading System. The first step in the process is to identify the gem type. After a thorough cleaning of the stone, it is held by its girdle (the widest point in circumference of a gem thus giving the highest concentration of color) over a neutral colored background. The gemologist looks at it face-up from about 18”, in strong, diffused, daylight-equivalent lighting, with the light source about 10” above the stone. The stone is then rocked back and forth up to a total of 30 degrees as the tone, hue and saturation judgments are made.
The gem’s tone, its darkness or lightness, is determined first. Next the gemologist estimates the overall hue. The hue reveals its colors in bright flashes of brilliance as the stone is looked at in its face-up position. Finally, the gemologist estimates the saturation level. If a stone does not exhibit brownish or grayish colors, it is considered to have a strong to vivid saturation. Saturation is less strong as browns and grays occur.
Saturation will also include judging where any additional colors in the stone occur, which may be a result of color change, pleochroism (the ability of certain gems to display two or more colors when viewed from different angles), color zoning, and windowing of less than 50% are noted. The term window, used in the context of colored stones and diamonds, refers to flaws in the stone’s cut.
When a stone is properly cut, the light should be reflected back out the top of the stone so you see only sparkle and light shining through the center, creating a "window" through which you can see straight through. All properly-cut gems have a window through their middle. Colored stones have much lower refractive indices than diamonds, therefore it is more difficult to achieve a perfect cut with colored stones than with diamonds. Colored gemstones require a special cutting style. Most cutters of colored stones will attempt to concentrate the color in the bottom of the pavilion so it reflects evenly through the stone.
Crystal Enchantments: A Complete Guide to Stones and Their Magical Properties
