How to Evaluate Colored Gemstone

By: Edward Bristol

Though most people think only diamonds are graded, there is also a set of criteria to evaluate colored gems. These are summarized in a grading report. A gem's value is primarily based on its color. Here are the basics you need in order to understand a colored stone grading report.

To understand color, remember that there are three primary colors (red, blue, yellow) and three secondary colors, which are the result of mixing primary colors (purple, orange, green). Nature often displays tertiary colors, such as red-orange, yellow-green, or blue-violet, which are a primary color mixed with a secondary color. When a color is mixed with gray, white, or black, we need the (overlapping) definitions of saturation, hue and tints.

Saturation expresses the attribute of perception of gray undermining the depth of color. If a stone were all gray it would be said to have zero saturation. Nature can not produce 100% saturation but if it could, there would 0% gray in the color. As a rule, the higher the saturation percentage is the more expensive the gem will be.

Hue is the actual color perceived whether red, purple, yellow, green etc. In these terms, white, black and gray have no hue. Some hues, like red, pink and blue, are (today) considered being more valuable than others (e.g. yellow or purple). This has and continues to change with fashion, over time and between cultures.

A color mixed with white is called a tint. A tint is lighter and less saturated than its original color. Generally speaking, the less tint, the better the hue, the higher the price. Exceptions are Padparadscha sapphires, Rhodolites and cornflower blue sapphires which are specifically characterized by the combination of hue and tint.

To describe a color technically, one could correctly say, as an example, blue (hue) mixed with 20% gray (saturation) plus a bit of yellow mixed with a lot of white (tint)". Such descriptions are hard to imagine and not very attractive. Therefore color professionals use more illustrative names such as "ivy green", "cornflower blue" or "salmon orange" in connection with attributes like "strong" or "vivid". Because these color names refer to something common and natural they express color in a more accessible way.

For the evaluation of gemstones, saturation, hue and tint are summarized in two criteria called color grade and tone. Color grade describes the strength of the main color compared to other colors visible in the stone. A 100% color grade in blue for example would imply that there are no other colors (like purple or violet) visible in the stone. If there was also no gray in the stone, we would have a 100% saturation with a 100% color grade, but such a stone has never been found. Some varieties (e.g. the Padaparadscha), which are defined by a combination of main colors (e.g. pink and orange), will receive a high color grade from the purity of the combined main colors, meaning the absence of other colors e.g. brown. Generally the rule applies: the purer the color the higher the grade. But grade is nothing without tone.

Any color grade has to be seen in combination with tone. Color tone varies from "very light" to "very dark". It is the amount of black or white mixed into a color. In the extremes, a colored stone could be white (light 5) or black (dark 95) with just a hint of color. Only grade and tone together describe color value sufficiently. A stone might, for example, show a rather pure blue, free of green or violet, but it might be of a very light tone thus the blue is less strong. Or it might, in the opposite, be of such a dark tone, that it appears rather black than blue. Gemstones with high color grades and light-medium to medium-dark tones fetch the highest prices. Grade and tone are expanded upon by applying the terms color zoning, clarity, brilliancy and depth.

Color zoning describes the way in which some stones show colors only in parts or layers. To describe the strength of this common but generally unwanted effect, we use four levels:

1.None: The color is equally distributed
2.Faint: One might see changes in color saturation
3.Gradual: The color weakens in some parts but not abruptly.
4.Visible: Stone has clear color patches or layers.

Other than clarity, which is judged with a 10x lens, color-zoning is described only as far as it is visible to the unaided eye.

The clarity of a stone can range from "Free of Inclusions" to "Excessively Included". Free of inclusions indicated that even under 10x magnification no inclusions become visible. A lightly included stone would show visible inclusions under a 10x magnification but rarely with the unaided eye. Moderately included have inclusions that may be seen with the naked eye. However, they do not dominate the stone. In a heavily included stone, the inclusions are clearly visible and influence the stone's appearance. If a stone is classified excessively included, it may be so flawed as to no longer be durable.

Brilliancy is the estimated maximum light a stone would reflect in one position under a spotlight. High brilliancy is, amongst others, the result of skilled cutting.

Depth is the height of a stone divided by its minimum width. The "ideal" range lies between 60% and 80%. It is mainly determined by the given shape of the rough stone. If the depth percentage is lower than 50%, the stone might be called shallow. A shallow stone with a light tone will find it difficult to maintain saturation. A stone with 90% depth and a dark tone on the other hand might black out.

Brilliancy and depth are joined in the "Cutting Grade" which also includes the general quality and precision of the lapidary's work and the finish of the stone. Here, a dealer will also mention any flaws or other weaknesses regarding the cut.

All quality parameters are then summarized into an overall grade. If you want to keep things simple you just have to look at the overall grade which is described by five levels:
oExcellent: Far above average and flawless. This quality is rarely seen in jewelry and is mostly acquired by collectors or long term investors.
oVery Good: Above average in all criteria with one or two minor flaws.
oGood: Average quality with strengths and weaknesses
oFair: Average quality with one or two obvious flaws
oPoor: Major imperfections

Ninety percent of the stones coming out of an average Sri Lankan mine will fall into the category "poor" and "fair", while only one individual in the whole lot might receive an "excellent".

Whatever gemologists, traders, miners, jewelers or grading reports say, you are the only one authorized to judge beauty. The less you like what everybody else does, the better for your budget! Your first evaluation of a gem will most likely be based on a photo. Remember that today there are many more excellent gem photos than good gems in the market. Always rely on a grading report from a reputable source to get the whole truth about any gem that you are considering purchasing.

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