How to evaluate colored gemstones:
Though most people think only diamonds are graded, there is also a set of criteria to evaluate colored gems. We summarized these in a grading report.
Read also about the right lens and the right light.
Cornflower or Centaurea:
Famous reference for
ere are the basics you need:
A gem's value is primarily based on its color. H
What is a color?
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.
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What is saturation?
Saturation expresses the attribute of perception of gray of the same lightness. All grays have zero saturation. Theoretically a 100% saturation means there is 0% gray in a color.
Nature does not come in 100% saturation, but the higher the saturation the more expensive the gem.
What is a hue?
Hue is color perceived to be red, purple, yellow, green etc., meaning 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.
What is a tint?
A color mixed with white is a tint. A tint is lighter and less saturated than a color without the addition of the white.
Generally speaking: the less tint, the better the hue, the higher the price. Exceptions are e.g. the padparadscha, the rhodolite or cornflower blue sapphire which are characterized by the combination of hue and tint.
How to describe a color?
You may describe a stone correctly as "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:
Color grade and tone.
What is a color grade?
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:
What is a color 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 framed by color zoning, clarity, brilliancy and depth:
Untreated 2.7 carat sapphire:
"Visible" color zoning but "Free of Inclusions"
Some stones show colors only in parts or layers. To describe the strength of this common but generally unwanted effect, we use four levels:
What is color zoning?
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.
What is clarity?
The clarity of a stone is commonly described as being from "Free of Inclusions" over "Lightly", "Moderately", "Heavily" to "Excessively Included".
Here is how we describe a stones clarity:
- Free of inclusions: Even under 10x magnification no inclusions become visible
- Very Lightly Included: Only a pro with a lens might find an inclusion, but maybe not.
- Lightly Included: Inclusions are visible under a 10x magnification but rarely with the unaided eye
- Moderately Included: One might see inclusions with the naked eye, but they should not dominate the stone
- Heavily Included: Inclusions are clearly visible and influence the stone's appearance
- Excessively Included: A stone might not be durable.
|4.24 ct Sapphire||3.87 ct Sapphire|
Photo: Two "Heavily Included" sapphires with "Visible" color zoning.
Remark on photo: While the yellow is too unevenly included, the blue has some "character", strong color but little luster, and it is not unattractive, though its value will be somewhere around only 5-10% of a similarly colored stone which is "Free of Inclusions".
What is brilliancy?
The estimated maximum of light which a stone reflects in one position under normal light conditions. High brilliancy is, amongst others, the result of skilled cutting.
What is depth?
It 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. Under 50% a 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 blackout.
What is cutting grade?
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 we will also mention any flaws or other weaknesses regarding the cut.
All quality parameters are then summarized into an overall grade:
What is "overall grade"?
If you want to keep things simple you just have to look at the overall grade which is described by five levels:
- Excellent: Far above average and flawless. This quality is rarely seen in jewelry and is mostly acquired by collectors or long term investors.
- Very Good: Above average in all criteria with one or two minor flaws.
- Good: Average quality with strengths and weaknesses
- Fair: Average quality with one or two obvious flaws
- Poor: Major imperfections
900 of 1000 stones coming out of an average Sri Lankan mine will fall into the category "poor" and "fair", while only one(!) might receive an "excellent".
BUT: 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.
Here are our prices ($/carat) for manually mined, individually cut, certified and untreated Ceylon gemstones based on the overall grade.
3. Gemstone sizes:
Read here to judge sizes for jewelry.
4. Gems and Photos:
Since your first evaluation of a gem will most likely be based on a photo:
Three shots of the same seven carat chysoberyl.
Note the color and visibility of the table feather. We would select
the left photo and mention a stronger green in the comment.
Today there are more good gem photos than good gems in the market. Read also about gem trading on the internet.
We do not color enhance or brighten or darken our photos. Our photos are made with a "normal" Nikon 5400 and every stone is shot individually. We never re-use old photos.
Left untouched, photos might show inclusions to dominate, weaken or strengthen a color, make a light stone darker or vice versa. If that is so, we mention it in our comment at the end of the grading report.
If you think a photo differs from our grading, you can assume that the grading is closer to reality than the photo. In any case our stones convince best in person.
Read an in-depth study here: Gemstones on Photo
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