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colored gemstone grading

Basics of Evaluating Colored Gemstones

The value of colored gemstones, like diamonds, depends on “the 4 C’s”.  Those are Color, Clarity, Cut and Carets.  Unlike diamonds, the value of a colored gemstone is effected most by the color.

Color is judged by three criteria: Hue, Saturation and Tone. Generally, stones with strong, pure hues are are more valuable.  For instance a slightly orangish red Ruby will be much less valuable than a stone that is pure red.  Saturation refers to the intensity of the color.  Highly saturated colors are very vivid.  Less saturated colors tend to be more brownish  or grayish.  Tone refers to how light or dark a color is.  For example, pink would be a light tone of red.  Stones with a very dark tone will be almost black.  The standards are different for different types of stones, but generally the best stones are ones with pure hues, vivid colors and moderate saturation (not too dark or too light).

Clarity takes into account the inclusions in the gemstone.  Inclusions are spots, fractures, imperfections, or anything that interferes with the passage of light through the stone.  Generally, a stone with fewer inclusions is more valuable.  Virtually all stones have some inclusions.  Some varieties tend to have more than others and so inclusions are more acceptable in some stones.  The best example is Emeralds.  They almost always have visible inclusions and so their value won’t be effected as much by inclusions as would an Amethyst, for instance, because there is a lot of very “clean” Amethyst available.  The Gemological Institute of America has classified all common gemstones into 3 types, based on how included they tend to be, and defined standard clarity grades for each type.  You can see their Clarity Grade Chart here.

Cut refers to how well faceted the stone is.  The stone should be well proportioned, and symmetrical.    The facet junctions (where the facets meet) should  be crisp and come together in a single point. The surface should be smooth and well polished.  The quality and the craftsmanship of the cut has a profound effect on the overall beauty of a gem and so has a significant impact on its value.

“Caret” refers to the weight of the stone.  Obviously a bigger stone is worth more than a smaller stone of the same quality.  With some varieties of gems, a larger stone will be more valuable “per caret” than a smaller one, because larger sizes are much more rare.  This is the case with sapphires.  A  3 caret sapphire will be worth much more than Twelve 1/4 ct. sapphires.  In other cases, like Citrine, where large sizes are readily available, the price per caret will be about the same, or may even be less for the larger stone.

Obviously, this is a complex subject.  In future articles we will look at some of the topics I’ve mentioned here in more detail.  But, hopefully this has given you an overview of the factors involved in evaluating a gemstone and some basis for comparisons when you are making your next buying decision.