Montana Yogo Sapphire Grading System


With the increase in values of Yogo sapphires over the last several years, it became increasingly evident to a several Montana jewelers that the traditional Yogo Grading system (comprised of AAA, AA, A and B) failed to account for the wide range of quality differences evident. Even if to some such differences appeared slight. Indeed, even the addition of a plus (+) and a minus (-) sign by some to the existing grades failed to give the specific and accurate quality ranking the trade was looking for. For example, we knew there were different classes of Yogo colors, different qualities of Yogo cut, etc. But no one had made any serious attempt to qualify or quantify them. Determining clarity in a Yogo was easy. But judging and ranking color and cut, and how these features combined to affect value, no one could seem to get a good handle on. The result was, Yogos of a lesser quality could sometimes end up commanding the same price or more per carat than a higher quality Yogo in the retail market place.

The purpose of this new Yogo grading system is to provide some reliable measure and certainty in grading and valuing Yogo sapphires. On the one hand, this new system is more complex than the previous simpler system of AAA, AA, A etc.. But in being more complex, its goal is to clarify and rank the specific features that determine actual Yogo quality. Thereby enabling both the wholesaler, jeweler and appraiser to determine Yogo value in a more consistent and reliable manner. On the other hand, while its complexity had the goal of providing an accurate quality ranking for Yogos, it was also designed to be simple. In other words, it was designed to be approachable, understandable, and usable by both the trade and public.

A little background on what quality ranking is can be helpful in understanding the new Yogo Grading System. When looking at the quality ranking system for any item, be it diamonds, coins, silver, furniture or even automobiles, every quality ranking system can be broken into roughly four categories. These four ranking categories can essentially be referred to in their simplest terms as: very good, good, fair, and poor. Now different systems may have their own unique terms to indicate this basic structure of very good, good, fair and poor. In the case of ranking the quality of diamond clarity, a “very good” category would be the VVS grades. Now it is not uncommon for each of these four grades to have sub-grades to indicate the high and low end of a particular category. Again, as an example, the “very good” grade for diamond clarity has a VVS1 and a VVS2 as its subcategories indicating the high and the low of the very good category. With this simple foundation, you can see how easy it is to extrapolate and come to a clearer understanding of quality ranking whether we are talking about coins, gems and jewelry or furniture. An educated eye with an understanding of the basic philosophy of quality classification (or ranking as appraisers often refer to it) can readily begin to group objects into their respective quality grades.

In the case of gemstones, including Yogos, there are four quality ranking categories. These four are carat weight, cut, color and clarity. While each category may be separately qualified and quantified, it is the interrelation of all four that goes into determining a value. Indeed, it really is the value that expresses the overall measure and composition of quality. Philosophically speaking, in that since, it can almost be seen as the overriding expression of quality or grade in itself. Since determining carat weight in a gemstone is pretty cut and dried, and easily measurable by a scale, we will focus on the other three and how they apply to measuring Yogo quality for both evaluation and valuation purposes.



Yogo sapphires are a singular and a highly unusual gemstone. Unlike other sapphires, Yogos are the only sapphire to be found in their original gem bearing dike. Sapphires throughout the world, other than Yogos, are found in old alluvial stream deposits; where they have been washed away from their original source. Whereas the majority of sapphires are formed in the hexagonal habit which can yield nice deep crystals. Yogos are found in the tabular variation of the hexagonal habit. Which means that Yogos are found primarily in very thin flat crystals that look like little tablets. Therefore, finding a Yogo crystal with good depth that will cut into a beautiful gemstone is quite rare in sizes over a half carat. Good depth combined with good cutting is critical to bringing out the beauty of a Yogo. The Yogo cut grade system focuses on and ranks those aspects tied to cut. It’s focus is on being able to measure and communicate in some meaningful and understandable manner the importance of cut to beauty in a Yogo.

There are two features of the Yogo cut grade. The first considers the over all depth of the Yogo, and this is broken up into four numerical categories. The second relates to those aspects of cut affecting the quality of the visual appearance to the eye, and these are broken into four alphabetical categories.

1st grade: the overall depth percentage of the Yogo is between 60.0% to 64.9%

2nd grade: the overall depth percentage of the Yogo is between 58.0% to 59.9%

on the low end and 65.0% to 69.9% on the high end.

3rd grade: the overall depth percentage of the Yogo is between 43.0% to 57.9%

on the low end and above 70.0% on the high end.

4th grade: the overall depth percentage is below 43%.

Each grade has an alphabetical qualifier to indicate both visual appearance taking in to consideration such factors as brilliance, extinction, windowing, polish, symmetry, alignment, etc.

A = Very Good: May have one or more of these features:

There is absolutely no windowing in the A category, and brilliance has to be at least 30% or better in a face up stationary position under the light source.

Symmetry: All facets are present, and facet alignment can range from good to very good. There are no open facet junctions. Table and culet should be centered. Polish can range from Good to Very good. Gemstone outline is very good, i.e. it is not out of round; or if a fancy shape, it is not out of balance.

B = Good: May have one or more of these features:

No windowing in the center (or under the table) of a round brilliant or oval although there may be some very slight windowing that might be evident through the crown facets in Yogos with depths greater than 65%..

Extinction is more prevalent, and brilliance is less than 30%. Brilliance may appear more muted or subdued and directional (i.e. improving in appearance when viewed as various angles and not necessarily straight-on and face up).

Symmetry can range from a low Good to Very Good. Table and culet may be very slightly off center, to centered and noticeable under the 10 power only. Noticeably off-center pavilions (including those readily discerned by the eye) cannot fall into this category, and should be placed in the fair category at best, and in the poor category if off center by 25% or more. There may be minor facet alignment and symmetry issues not apparent to the naked eye, and not affecting the overall visual appearance of the Yogo. Gemstone outline is at least a good, (may be very, very slightly out of round at 1.50 mm or less) up to very good.

C = Fair: May have one or more of these features:

Some windowing evident through the table or crown from 10% to 30%. Extinction usually comprises a major portion of the gemstone. Brilliance is below 30%. Symmetry: table and culet could be noticeably off center under 10 power. There may be missing facets. This should only be evident under 10x, but not to naked eye. All mixed cuts with slight bulges may fall into this category or lower. There may be large extra facets on the crown that break the symmetry and alignment of the facets. There may be large open facet junctions. Stone outline can range from fair (namely, slightly out of shape) to very good. In the case of round brilliants, the gemstone is more than 1.5 mm out of round on round brilliant cuts. There maybe naturals that break its outline, or extend onto the pavilion or crown. Any round brilliant cut or fancy cut, with a “noticeable-to-the-naked-eye” break to its outline, or without a balanced outline, will fall into the C or fair category.

D = Poor: May have one or more of these features.

Major windowing occupying 30% of the gemstone or more. Brilliance often confined to the edges under the crown facets. If the Yogo is mounted, windowing may be muted and appear as extinction (as black rather than clear). The quality of the symmetry can range from poor to very good. Stone outline can range from poor (badly out of shape) to very good. Table and culet off center. Yogo may be noticeably flat, or cut with a large bulge to the pavilion.

Explanatory Example of the Yogo Clarity Grade.

An example of the cut grade is “1A”. This indicates the Yogo was cut with a desirable depth, proper proportions, and overall care in the cutting to result in the best eye appeal.


Yogo Color Grade

Yogos receive their distinctive cornflower or violetish blue color from the presence of titanium. Because it gets its color of blue primarily from titanium rather than iron, unlike other sapphires (excluding some Kashmir and heat treated blue sapphires) well made Yogos are just as lively under artificial light as in daylight. Where most sapphires appear dead in artificial light, a well made Yogo appears a lively electric blue. At present we have been able to discern five basic color categories of Yogos. As with all things in nature, there are exceptions. However, after reviewing a large sample group of over 100 Yogos, these categories seem the most defined and able to accommodate the vast majority of Yogo color samples.

There are two features of the Yogo color grade. The first considers type of color in the Yogo, and this is broken up into five numerical categories. The second qualifies the quality of color in each category and ranks it as to its visual appeal. These are broken into four alphabetical categories. So the Yogo color grade will read, for example as 1A. The 1 refers to the color category, and A refers to the quality of that color in the 1 category.

It must be emphasized that a category 1 color is not better than a category 3 or 5 color. The color categories are just different. One can still have very good quality color Yogos in each of the five categories. From the standpoint of quality ranking, it is the alphabetical grades that are the most important.

The Five Categories of Yogo Color.

Yogo’s should be graded loose with their tables down with the color being viewed from both the top and side of the pavilion. A daylight fluorescent bulb is best. Yogo master stones are helpful when separating category 1 and 2.

Quality of Color ranges from A to D or very good to poor

Category 1. (Royal or Imperial Blues) very minimum to almost no trace of violet evident to the naked eye)

Category 2. (Cornflower Blues) blue with noticeable traces of violet to the naked eye.

Category 3. (Cornflower Violets) violet seems almost equal to or is stronger than the blue. Mounted, in artificial light, the Yogos may have a strong violet-blue appearance face up. Often this violet disappears when the Yogo is exposed to daylight.

Category 4. (Lilac Yogos) Distinctly purple to red in artificial light. Please note that many of these Yogos will often show a distinct and a strong change of color to blue when exposed to daylight. This alexandrite-like effect must be measured separately. The finer grades of Alexandrite-like Yogos are highly desirable. But to be considered a fine grade the purple color must be very good, and the color change must be strong, immediately noticeable, and the quality of the color of blue that results must be very good. The point to remember is that not all change of color Yogos command a high value. Red Yogos or rubies are very, very rare; and in the better grades, command a high value in nearly every size range. They are rarely found, and rarely make it into the retail market.

Category 5. (Greenish Blue Yogos to Greenish-violetish Blue)

For years the trade has been under the assumption there is no green dichroic color evident in a Yogo. Such is not the case. Although not common, you can have green dichroism in a Yogo. Moreover it can appear along with violet as a modifier to the Yogo’s primarily color of blue. Now, no “all green” Yogos have been found. But definitely you can have green as a modifier to the primary blue color in Yogo. Indeed, in the both the darker and finer A categories, the presence of green (even if slight) and as a modifying color (even with violet) can result in a truly beautiful Yogo. So the presence of green is far from an undesirable feature. However, most greenish blue Yogos tend to be very pale in color and are not nearly as attractive as the other Yogo colors. But this feature would be noted and reflected in the Yogo quality of Color ranking (see below) which is our next subject to discuss in describing and ranking Yogo color.

Quality of Color

The second part of the color grading system ranks the quality of color in each category as to its visual appeal. These are broken into four alphabetical categories from A to D or very good to poor. The quality of color is very much influenced by the proper depth and quality of cutting. In this part, we are ranking not only the overall quality of color but the also the Yogo’s overall visual appearance and appeal to the eye. In this instance, we are employing an aesthetic judgment which in all fairness for disclosure purposes (and which Kant pointed out) has elements of the subjective and is somewhat governed by and limited to personal taste. However, there are guidelines (i.e. criteria provided below) that this system uses to prevent the license of someone calling a lower grade Yogo a much higher grade because “it is their opinion“ or that “it looks better to them.”

Class A. May have one or more of these features:

Dark Yogos with good to very good brilliance. Very Dark Yogos with brilliance between 25 and 30%.

Class B. May have one or more of these features:

Medium to Medium dark Yogos with good to very good brilliance. Dark to Very dark yogos with brilliance under 25%

Class C. May have one or more of these features:

Light Medium to Medium Yogos. Light medium can have fair to very good brilliance. Medium Yogos showing poor to fair brilliance. Many slightly windowed to moderately windowed Yogos in all color tones from Very Dark to Light Medium begin to fall into this grade and lower.

Class D. May have one or more of these features:

Pale to Very Dark Yogos, heavily windowed, brilliance may be seen around the sides of the gemstone. Yogos may show no windowing at all but with 95 to 100% extinction and little to no brilliance. In other words, a dead looking gemstone.

Explanatory Example of the Yogo Color Grade.

An example of the color grade is “1A”. This indicates the Yogo is in the color Category 1 which is a Royal or Imperial Blue Yogo color, and is a Class A indicating it has the best eye appeal for that particular color category.


Yogo Clarity Grade

The Yogo clarity grading system pretty much mirrors the GIA clarity grading system for sapphires. However, in this system, clarity grades are assigned numerical values with the following descriptions:

0 – Internally flawless. There are no visible internal or external imperfections under 20x when viewed either through the crown or pavilion.

1 – Very minute imperfections, extremely difficult to see. Usually only seen under 10x power through the pavilion of the gemstone, and are not visible through the crown.

2 – Very minute imperfections, very difficult to see under 10x either through the crown or pavilion.

3 – Minor imperfections somewhat difficult to see under 10x.

4 – Minor imperfections somewhat easy to see under 10x.

5 – Noticeable imperfections easy to see under 10x.

6 – Noticeable imperfections very easy to see under 10x. May be slightly visible through the pavilion to the naked eye, but cannot be seen with the naked eye through the crown.

7 – Noticeable imperfection visible at 10x. May be seen with some difficulty by the naked eye when viewed through the crown.

8 – Noticeable imperfections when viewed under 10x and easily seen by the naked eye.

9 – Obvious imperfections prominently visible both under 10x and to the naked eye. Imperfections in this grade may also affect durability



Finally, the value ascribed to any Yogo will take into consideration the weight, as well as the quality of color, cut and clarity. It is by measuring all four of these ranking factors together that value or price is determined.

Please Note: Certain elements of this system may change or become more refined as more data is gathered, examined and weighed. The ultimate guideline is improving the system to reflect accuracy, fairness, impartiality and balance. To that extent, we do reserve the right to make changes when and where necessary; where either the existing facts, or the consideration of new facts and data warrant it.

If you have any further questions, you are welcome to contact me at:

Anthony P. Kelson


Copyright 2008 © Anthony P. Kelson