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Home Gemological Information Gemstones



On the Gemological Institute of America’s grading system, color is described by: HUE, the spectral color, such as red, blue, yellow, etc.; TONE, how light or dark the color is, from Very Light [2] to Very Dark [8]; and SATURATION, purity of color, from Grayish or Brownish [1] to Vivid [6]. In describing a gemstone’s color the GIA first names the modifying hue with lower case letters and then the primary hue with a capital letter, followed by the rating for tone and saturation For example, a medium-dark toned, strong-colored, very slightly bluish-green emerald would be annotated vslbG 6/5. Since the best achievable color for one gem variety may be different from another variety of gem with the same basic color, the color is then ranked on a desirability scale of 1-10, with 10 being best.


The GIA ranks gemstones into three clarity types. TYPE I gemstones are expected to have few if any inclusions, e.g., topaz or green tourmaline. TYPE II gems are usually included to some extent, e.g., ruby, and peridot. TYPE III clarity gems are almost always included, e.g., emerald and red or pink tourmaline. The gemstone is rated for clarity against stones of their same type, so that a VS grade, Type I stone (such as a green tourmaline) would appear noticeably "cleaner" than a Type III emerald of the same VS grade. Since clarity is not as critical a component of the beauty of a colored gemstone, the clarity grades are broader than they are for diamonds.


The colored gemstone is described by Shape, then graded for Cut. Cut refers to Proportion and Finish. Proportion is composed of face-up outline, profile, and brilliance. Finish is composed of polish, symmetry, and appropriateness of facet number. Face-up Outline refers to the symmetry of shape from the top. Profile refers to the balance of the gem as viewed from the side: the depth percentage, the crown height ratio, the amount of bulging adding needless weight to the bottom, the table proportions, and the suitability of the girdle (edge of the stone) for setting. Brilliancy, the return of light from faceted transparent gems, is expressed by subtracting the percentage of windowing (see-through effect) and extinction (dark, unreflective areas) from 100%. Cabochon-cut gems are judged primarily on symmetry and polish.

All gemstones other than diamonds are considered colored gemstones. Over one hundred varieties of gemstones are routinely encountered, and each variety has its own subtle differences in grading. This, and variations in the ways different individuals perceive color, make colored stone grading even more subjective than diamond grading. The goal of the gemologist is to communicate as accurately as possible the characteristics that are being described. This is carried out by making a word picture of the stone referring to standard comparators and charts, and by plotting a diagram of the gemstone when appropriate.

Gemstones are commonly treated by heating, oiling, irradiating, filling, dying, etc. Some treatments are accepted practices of long standing. Others are considered deceptive treatments and have an extreme negative impact on value. Country of origin, if verified, can have a large positive impact on value providing the stone is typical of its region. The GIA has not yet developed grading nomenclature for non-transparent gemstones such as jade, and phenomenal gems: chatoyant gemstones such as cat’s-eye crysoberyl, astriated gems such as star sapphires, adularescent gems such as moonstone, and play-of-color gems such as opal and fire agate. However, the color and cut can be described by GIA nomenclature, and the clarity and phenomena described and ranked.