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Diamond Grades GIA (Gemological Institute of America)

Colorless

Near
Colorless

Faint
Yellow

Very
Light Yellow

Light
Yellow to
Yellow

Diamonds
The most precious diamonds are generally colorless, like pure sparkling water. However, most natural diamonds have traces of nitrogen in them which turn the diamond yellowish or brownish. The nitrogen atoms displace carbon atoms in the lattice structure of the crystal. This absorbs some of the blue spectrum from the light, leaving yellow. The Gemological Institute of America has developed standards and grades for color in diamonds. As illustrated above, the grades run from D to Z. Only a well trained jeweler with special lights and tools can assess these subtle differences in grades.

Some diamonds have other distinct colors caused by inclusion of various elements or natural irradiation. Slight discoloration generally lowers the value, whereas vivid coloration is far more rare and places them in the category of Fancy Colored Diamonds. Examples would be the Deep Blue Hope Diamond, Pink Grande Conde Diamond, and the rarest diamond in the world, a Fancy Deep Blue-Green called the Ocean Dream Diamond.

Color Enhancements
Since the turn of the century people have been altering the appearance of naturally occuring stones. In 1904 it was discovered that diamonds could be turned green by subjecting them to irradiation. Rough Tanzanite gemstone is brown in nature, and the beautiful blues and purples of the finished stones are gotten by treating it with heat. Typical treatments of stones include HTHP (High Temperatur/High Pressure), CVD (Chemical Vapor Deposition), annealing, dyeing, and irradiation.

Natural Fancy Color Diamonds are significantly more valuable than comparable treated or synthetic stones. Treated and synthetic diamonds can be beautiful in their own right,  however, the origin of their color should be fully disclosed by the seller. Ethical practice dictates that synthetic gemstones and treatments be fully disclosed to consumers. In the US, the Federal Trade Commision regulates disclosure. Unfortunately,  truth in reporting does not always happen. If there is a question, labs can determine what a stone truly is, and what has been done to it.

Grading Colored Gems
Colored gemstones are graded in the same fashion as diamonds. That is, they are evaluated on Color, Clarity, Cut, and Carat Weight. Colors are described by three factors - Hue (blue, red, etc.), Saturation (strength of the hue), Tone (darkness). Highest values generally go to stones with strong rich colors and pure hues. For example, a pure red ruby will be worth much more than one that is more orangish. Although color is one of the hardist features to evaluate, color grades of gemstones has been standardized by groups such as GIA.

Sources and Resources:
A Consumer’s Guide to Gem Grading – International Gem Society
About gemstone enhancements – Wikipedia.org
American Gem Society

American Gem Society Laboratories
Colored Gemstone Grading, Grading Reports & Labs - Khulsey.com
Diamond Color Enhancements - Wikipedia.org
Fancy Colored Diamonds - Khulsey.com
Jewelry Lab Certificates and Appraisals - Novori.com
Natural Color Diamond Association
Synthetic Diamonds - Khulsey.com
The Difference Between a Jewelry Appraisal
    and a Lab Certificate - About.com
  

Fancy colored diamonds


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