There is no grading system for turquoise. Any determination by grade is entirely subjective and dependent upon the individual opinion on the grade. This presents the incentive for manipulation by those who wish to increase the price of their turquoise by claiming a higher grade. There have been efforts to create some measurable objective form of grading. The first was by Robert Biesecker who proposed a system in the 1977 Turquoise Annual of the International Turquoise Association (Robert's Rules). These have been simplified for this proposal. There was a short lived effort, led by Dennis June on Facebook, to develop some form of turquoise grading but this effort was unsuccessful. My suggestion of a proposed grading method, presented to that grading group in 2015, was rejected but no other proposal was developed.
Grading has never been accepted. Although collectors and consumers are favorable to a method of grading, miners and dealers, presumably fearing a potential reduction in revenue from lower grades for their turquoise, have been steadfastly opposed. I feel this is a misguided attitude and that an accepted grading method would inspire greater consumer confidence and increased sales of turquoise. Price would be a natural arbiter in the marketplace. While a buyer may desire gem grade or high grade, the geometric escalation of price by grade would necessarily allocate buyers, by budget, into lower grade but still very beautiful turquoise. Also the extreme rarity of true gem grade means there is never much on the market. This is the de facto condition of the market today.
Modified Robert's/Ryan Rules of Grading Turquoise (R 2 Grade)
While keeping the process of Robert’s Rules in place R 2 simplifies the system and removes ancillary criteria such as demand and ratio of backing. Demand is a function of the marketplace and, while it will impact price, should not be an influence in the grading process. While excessive backing will add weight it does not change the grade of the stone.
R 2 has three basic grading categories each with its own set of grading guidelines. This rating system applies only to natural turquoise that has not been treated in any manner either for hardness or color. Treated turquoise, which comes in a range of quality, should be judged on its own because the addition of plastic or other substances injected inside turquoise will artificially alter the appearance of the stone. R2 grade does not apply to treated turquoise considered stabilized, enhanced, color shot or compressed block. Backing for stability is permitted as long as the depth of backing does not contribute a significant percentage of the total carat weight. Surface treatment, including filling of pits and fissures, is permitted but will result in lower grading for matrix.
The three categories for grading are:
1. Color. Measured by hue and saturation. 2. Hardness/Density. Measured by resistance to abrasion (Moh scale) and mass per unit. 3. Matrix. Measured by hardness, pattern, and bonding of turquoise with the host matrix.
Each turquoise is measured by a numeric measurement, out of 100, for each category, following recommended guidelines for grading, based upon a total allocation of 100%.
Color – 50% Hardness/Density – 25% Matrix – 25%
For turquoise with no matrix the allocation of grading for matrix will be divided between color and hardness.
No. 8 turquoise
Example. Gem grade spider web.
Color. 92 out of 100 x .50 46
Hardness/Density. 95 out of 100 x .25 23.75
Matrix. 94 out of 100 x .25 23.50
Total R 2 grade Gem Grade GG 93.25
For solid color turquoise of similar quality.
Color 94 x .625 58.75
Hardness/Density 95 x .375 35.6
Total R 2 grade GG 94.35
General grading qualification.
>91-100 GG Gem Grade >85-91 VHG Very High Grade >80–85 HG High Grade >75-80 HMG High Mid Grade 50-75 MG Mid Grade <50 NG No Grade
Since natural untreated turquoise comprises a relatively small percentage of total turquoise production (perhaps less than 20% and in some mines as little as 2%) R 2 assumes that the majority of the material will be mid grade or higher. Turquoise that grades below a score of 50 is not considered for a grading designation under these rules. Because any grading system will have a subjective element multiple grading by different graders is preferable. Collectors, miners and dealers should have independent grading made on their own turquoise.
Any method of grading is subject to subjective bias. It is recommended that no grade be given without at least three independent grading appraisals with the final grade the average of all grades. Photographing turquoise is very difficult to capture true color, hardness and matrix and photos may be manipulated. Whenever possible grading should not be done from photos although in practice this will be difficult and costly. Grades made from photos should rely on honest representation without digital alteration and recognized as such.
General Grading Guidelines
Backing. It is recognized that backing may be needed to insure the integrity of the turquoise cab. The amount used should be the minimal amount that will meet this objective without adding unnecessary weight to the finished cab. Buyers do not wish to pay turquoise prices for plastic steel. In general the backing should not exceed 20% of the total height of the cab. For most cabs this will limit the backing to a maximum of 2mm.The exception to this would be very thin naturally occurring turquoise, often higher grades, where even a minimal thickness of backing would exceed this guideline.
Color (Definitions credit to colorcube.com)
Color is the principal aspect of turquoise and receives the highest weighting for scoring. The hue for turquoise will be green to blue or a combination, and usually the deeper the color saturation the higher the grade. With turquoise hue defines pure color in terms of green, or blue. Hue also defines mixtures of two pure colors like "blue-green".
Hue: This is what we usually mean when we ask "what color is that?" The property of color that we are actually asking about is "hue". For example, when we talk about colors that are red, yellow, green, and blue, we are talking about hue. Different hues are caused by different wavelengths of light. Therefore, this aspect of color is usually easy to recognize. Hue Contrast - strikingly different hues Hue Constant - different colors, same hue (blue)
Saturation: Related to chromaticity, saturation tells us how a color looks under certain lighting conditions. For instance, a room painted a solid color will appear different at night than in daylight. Over the course of the day, although the color is the same, the saturation changes. This property of color can also be called intensity. Be careful not to think about saturation in terms of light and dark but rather in terms of pale or weak and pure or strong. Saturation - same intensity, different hues. Saturation Contrast - various levels of fullness, same hue.
Saturation defines a range from pure color (100%) to gray (0%) at a constant lightness level. A pure color is fully saturated. From a perceptional point of view saturation influences the grade of purity or vividness of a color/image. A de-saturated image is said to be dull, less colorful or washed out but can also make the impression of being softer.
Hardness is measured by the Moh scale and will generally range from 5-7 for turquoise. In the absence of measurement either by displacement tests or calculating by weight (mass) divided by volume, much scoring will depend upon the graders knowledge of a wide range of turquoise. Perhaps the best criterion for grading hardness is the ability of the turquoise to take a fine polished finish.
Higher ranking will be applied to distribution of turquoise in a balanced pleasing manner with hardness (the ability to hold a mirror like finish) and that is bonded with the turquoise in a manner that gives depth and translucence to the finished cab. This measure will be more subjective and depend upon the graders familiarity with a broad range of turquoise and will be relative to specific mines and the variety of different turquoise found in each. In this context rarity should be considered in the grading so that for two stones of near equal grading the more rare turquoise should receive a somewhat higher numeric score.
Grading remains as disputed a topic today as it was in 1977 (or in 2015). Yet while many will argue as to the method of grading or even if such a system is either possible or desirable, I would hope that all would agree that a basic grading system, however limited and lacking in any official capacity, is preferable to no rules of grading which allows misrepresentation to the detriment of everyone.
I welcome interested parties to experiment with R 2 by using the system to grade different turquoise from a variety of mines and of different qualities. Perhaps over time this grass roots effort, unencumbered with any official oversight or vested interest, will gain in usage and become a de facto standard. There is no penalty for trying.
Mike Ryan II March 2015
Revised January 2021