Colored Gemstone Grading
Due to the uniqueness and dynamic variation of nature, colored gemstones do not have a standardized international grading system in the same manner that has been established for diamonds. However, there are specific parameters considered integral to the valuation of gemstone quality.
The most important value factor of colored gemstones is the color itself. Color is a combination of three components: hue, tone, and saturation. Hue is the overall body color of a gemstone. Tone rates the brightness or darkness of a gemstone. Saturation is the intensity of color in the stone. In some gemstones, a certain combination of hue, tone, and saturation are highly desired and may be referred to by known trade terms of that color, for example “pigeon's blood” in ruby.
Clarity grading for colored gemstones is dependent upon stone type, a classification system that addresses different crystal growth characteristics. Clarity grading is only applicable to transparent gemstones. Some gemstones are commonly included, while others are relatively free of inclusions at 10x magnification. By grouping gemstones based upon typical clarity expectations, one is able to evaluate rarity. For example, in the beryl species, aquamarine is usually relatively free of inclusions, while emerald is often heavily included, thus making an eye-clean emerald much more rare than an eye-clean aquamarine. There are three categories for stone type.
Type I gemstones typically grow free of most inclusions at 10x and are commonly eye-clean. Type II gemstones typically grow with minor inclusions, some that are eye-visible. Type III gemstones typically grow heavily included, with almost always eye-visible inclusions. Sometimes the inclusions present in a stone help contribute to their beauty and attractiveness in the gemstone market.
Carat measurement for colored gemstones follows the same form as for diamonds, in which a carat is measured by 100 points in one carat. All other factors equal, the higher the carat weight, the higher the value of the stone.
Cut assessment in colored gemstones is subjective to many factors, the most notable based on crystal species and variety, rough crystal shape, market preferences, and cutting yields. Colored gemstone cuts may display much variation, as cutters aim to maximize carat weight and optical properties such as color saturation and phenomena. Important considerations for evaluating cut include: proportions, symmetry, and finish. A well proportioned gemstone maximizes brilliance and scintillation, a symmetrical cut displays fine craftsmanship and attention to detail, and a high quality finish will have aligned facets, absent of polishing marks.
Advances in modern technology have allowed for cutters to have precise control over mathematical angels known as precision cuts. Conversely, native cuts may prioritize the final weight of the stone over otherwise more advantageous angels. These cuts may include some undesirable features such as windowing, in which areas of light pass through a gemstone without brightly reflecting back to the eye, making for a desaturated area of color, and extinction, in which dark areas of a gemstone occur due to light not being reflected back to the eye. There are also fantasy cut and custom cut gemstones that result in fanciful shapes or faceting, unrestrained by conventional styles. Sometimes faceting is not used at all, and a gem is simply polished to a degree of curvature, known as cabochon cut.
The complexity of colored gemstone cutting limits the possibility for a universally accepted standard, however we are committed to providing our consumers with as much information as possible. If there is additional information you would like regarding a specific item, please do not hesitate to send us a request. We are happy to provide as much information as reasonably possible.