Gem Stone Grading
Gemstone grading though most people think only diamonds are graded, there is also a set of criteria to evaluate colored precious gemstones.
A gemstones value is primarily based on its color. Here are the basics you need:
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.
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 considered being more valuable than others 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.
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”.
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:
What is color zoning?
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.
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.
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”. The GIA ranks gemstones into three clarity types. TYPE I gemstones are expected to have few if any inclusions, e.g., topaz or green tourmaline. TYPE II gems are usually included to some extent, e.g., ruby, and peridot. TYPE III clarity gems are almost always included, e.g., emerald and red or pink tourmaline. The gemstone is rated for clarity against stones of their same type, so that a VS grade, Type I stone (such as a green tourmaline) would appear noticeably “cleaner” than a Type III emerald of the same VS grade. Since clarity is not as critical a component of the beauty of a colored gemstone, the clarity grades are broader than they are for diamonds. All gemstones other than diamonds are considered colored gemstones. Over one hundred varieties of gemstones are routinely encountered, and each variety has its own subtle differences in grading. This, and variations in the ways different individuals perceive color, make colored stone grading even more subjective than diamond grading. The goal of the gemologist is to communicate as accurately as possible the characteristics that are being described. This is carried out by making a word picture of the stone referring to standard comparators and charts, and by plotting a diagram of the gemstone when appropriate
IF – Internally Flawless. The Gem has no inclusions even under 10X magnification and exceptional brilliance.
VVS1 – Extremely Fine Quality Gemstones are better than eye clean (i.e., have no visible inclusions) and have very few microscopic inclusions which are very difficult to detect under a 10x magnification loupe. Typical inclusions: pinpoints, very fine needles, tiny hairline feathers, minor color zoning, very faint clouds, and percussion marks.
VVS2 – Extremely Fine Quality Gemstones are better than eye clean (i.e., have no visible inclusions) and have very few microscopic inclusions which are difficult to moderately difficult to detect under a 10x magnification loupe. Typical inclusions: small included crystals, liquid inclusions, numerous fine needles, and small feathers.
VS1 – Very Fine Quality minor inclusions are only visible under a 10x magnification loupe but not visible to the naked eye. The beauty of the gem is not diminished in anyway.
VS2 – Fine Quality Minor inclusions are only visible under a 10x magnification loupe but not visible to the naked eye. The beauty of the gem is not diminished in anyway.
SI1 – Good Quality. Lightly Included. Some minor visible inclusions are present to the naked eye but generally not visible to the friendly observer. Typical Inclusions: included crystals, large
fingerprints, chips, feathers, considerable color zoning and dense clouds.
SI2 – Medium to Good Quality. Lightly Included. Inclusions are moderately easy to detect with the naked eye. Typical Inclusions: included crystals, large fingerprints, chips, feathers, considerable color zoning and dense clouds.
I1 – Included 1 The inclusions are very obvious and they have a moderate negative effect on the over-all appearance or durability of the gemstone. Characterized by inclusions that have a negative effect on either appearance or durability
I2 – Included 2 The inclusions are very obvious and they have a severe negative effect on the over-all appearance or durability of the gemstone, a severe effect
I3 – Included 3 The inclusions are very obvious and they have a sever negative effect on both the over-all appearance and durability of the gemstone. Inclusions are often large and prominent to the unaided eye
The Four C’s Of Gemstones
Colors are described by three factors:
Hue, (red, green, blue, yellow, brown, black, gray, white, orange, pink, purple, & clear)
Saturation, (strong or pastel, red or pink.)
Tone, (clear to dark.)
Generally speaking, highest values go to stones with pure hues and strong rich colors. With high value gems, subtle variations make a significant difference in price. For example, a slightly orangish ruby will not be worth nearly as much as one that is pure red. Most people cannot see the difference, but to the expert grader it is significant. On moderate priced gems, color has less affect on value. For example, tourmaline comes in every possible color. Unless it is an exceptionally pure green, red or pink, they are all about the same value. It is the other factors of size, clarity and cutting that determine the gem’s value. Diamonds are graded on how close they are to being colorless. There are three grades of “colorless” that only vary by subtle differences in transparency. Then there are four grades of “white.” These are gems that will appear colorless when set in jewelry. It takes an expert in a laboratory setting to distinguish between these grades, but each represents a change in value. The color that is best for you is a personal matter. Strong, bold colors correspond with strong personalities; others prefer something softer and brighter. The color of your complexion and the color of the clothing you prefer also have a significant effect on your gem choice. To find what colors are best for you, look at a lot of gems. You will find that subtle variations in color can have a significant affect on both how they look on you and your emotional reaction to them. If you are a connoisseur of fine gems, the higher grades are significant. On the other hand, the average consumer will find just as much beauty in the more modestly priced stones. Remember, quality does not mean better, it means rarer. Read More
Take note of the clarity of your gem using your magnifying glass or jeweler’s loupe. The clarity refers to the gem’s internal purity and the type and quantity of inclusions in the gem. Inclusions are features that can be seen inside the particular gem. Sometimes, these inclusions can be seen with the naked eye and other inclusions can only be seen under magnification.
Gemstones contain a wide variety of “inclusions.” In a faceted gem, an inclusion is defined as anything that will interfere with the free passage of light. They can be little bits of minerals, hollow areas, or fractures. As with color, tiny differences which are only apparent to the grader have a significant affect on value. The best examples of this are diamonds. There are several grades where the inclusions are invisible to the naked eye and have no affect on the beauty of the stone. Yet the difference in value, between something that is very difficult for an expert to find with 10 power magnification and something that is easy to find with magnification, is substantial. Most colored stones are simply graded “eye clean,” (meaning that the inclusions are not visible to the naked eye,) slightly, moderately, or heavily included. Gems with eye visible inclusions are always lower in value, but the change is not applied equally. There are three classes of colored stones, those that are “usually clean,” those that are “usually included,” and those that are “almost always included.” Emeralds fall into the last category. Their clarity cannot be compared with other gems. If you want an emerald without any eye visible inclusions, you are limited to small stones. If you want a larger emerald, you will have to accept a certain amount of inclusions and find its value in the color. If this does not appeal to you, then look at other green stones, like tourmaline and diopside. Their color can equal that of fine emeralds, without the inclusions, and at a much lower price. Now your choice is between having the name “emerald” or the premium color. Eye visible inclusions always have an affect on value. Sometimes this can be used to your advantage. I remember one young woman showing off her engagement ring. She had me look real close to see three tiny black dots. By accepting those small inclusions, which could not be seen from more than six inches away, her fianc� was able to afford a much larger diamond. You will find just slightly visible inclusions in colored stones as well. Even some stones that are moderately included, like dark garnets, will look fine from the distance jewelry is usually viewed from. It is up to your personality to determine if it is going to affect the joy you get from the stone. While beauty is in the eye of the beholder, you need to pay special attention to fractures and veils. They represent a weakness in the gem and are prone to breakage. Earrings, pendants and brooches do not receive much abuse, but ring stones are subjected to a constant onslaught of bumps and bangs. If you intend a gem for a ring stone, you should be particularly cautious of a weak gem. Finding these types of inclusions in a gem, and determining their affect on its integrity, is difficult for the lay person to do. The old advice, “If you don’t know your gems, know your gemologist,” applies here.
View the cut of your gem with your jeweler’s loupe. Gems that have a better cut are given a higher gem grade. If the cut is done well, it will reflect light better. The cut of a gemstone, the workmanship that went into fashioning it, is one of the most important factors in its appearance. It is also one of the most difficult factors for the non-professional to judge because of the number of variations involved. The first thing to do is to look at the shape of the stone. Some gems are cut “freeform” but most are intended to be a regular shape. If so, look at the symmetry. Does it bulge here or there, or is it symmetrical in all directions. Look at the stone from the side and the ends. Again, it should be symmetrical in all directions. If not, it is up to you to determine how much it will affect your appreciation of the stone. When considering a gem, insist on inspecting it with magnification. Look at areas where light is being reflected from the surface. They should be smooth and mirror like. If you see pitting, scratches, or dull areas, the gem is not well polished. It may look good in the store, but someday you will compare it with a well polished gem and be disappointed with your purchase. If it is a faceted gem, look at the facets junctions also. On a well cut stone, they will be crisp and come together in a single point. You may see facets that are slightly rounded and not quite meeting where they should. Just how far off they are will affect the brilliance of the gem. The other factors in a faceted gem are too complex for the lay-person. However, you can get a good idea of the cutting quality simply by comparing it with other gems. When you do this, make sure you are comparing similar stones. An amethyst will never have the brilliance of a topaz and dark stones will not be as bright as light colored ones. When comparing like stones, what you want to look for is the overall light return, the brilliance and sparkle of the gem. It is entirely possible to look at two gems with the same size and coloring, but one having much more brilliance and sparkle than the other. This is the result of cutting. One thing to pay special attention to is “windowing.” That is where light passes straight through the center, rather than being reflected back. It is easy to spot; the center will be much lighter than the outside of the gem and have no flashes of light. Some windows are small; others are quite large and hideous. This is another example of something that might look good in the store, but someday you will compare it with a well cut gem and be disappointed with your purchase. Cabochons are easier to judge. Begin by checking the polish under magnification. Then hold the stone a short distance from your head and rotate it slowly. Notice how the light passes across the surface. On a well cut gem, it will flow smoothly from one side to the other. If it is poorly shaped the light will not flow smoothly, but snake across the surface. Surface irregularities and poorly polished areas will also show up this way.
Pay attention to the carat weight of your gem. The weight of a gem is expressed in carats, and larger gemstones are less common than smaller ones. Simply put, larger stones are less common than small ones. Hence, they demand a higher price per carat. For example, a quarter-carat topaz may cost $60 per carat, or $15. A half-carat topaz, (with the same color, clarity and cutting grades,) might cost $100 per carat, or $50. A full carat topaz would cost $200. Choosing the right size is a personal matter. For the bold, dynamic individual, a large gem mirrors their personality. On the other hand, small stones are better suited to someone with delicate and feminine tastes. Most people will fall in between these two extremes. When budget is a strong factor, smaller stones have a significant advantage. Not only do they cost less per weight, the amount of gem you see is disproportionate to their size. The reason is that volume goes up faster than the outside dimensions. For example, a half carat, round diamond measures 5 mm in diameter, a carat diamond 6 mm, and a full carat 6.5 mm. From a casual observation, the half and carat stones, or the and full carat stones look to be about the same size, but the price difference can be considerable. Small gems are often clustered to give the illusion of more gemstone. Seven 1.6 mm diamonds, set close together, will take up as much space as a whole carat diamond. If set on white gold, it is hard to distinguish the separate stones, hence these are often called “illusion settings”. While these seven stones approach the eye appeal of a one carat diamond, they only weigh .14 carats. Considering that the price per carat is also much lower, the cost difference is significant. A cluster ring would be in the hundreds of dollars, rather than the thousands. Colored stones are often clustered for the same reason: you get a lot more visible gem for less money. If your budget doesn’t allow for your first choice in a gem, consider a setting with several smaller gems. With the right piece, you may find just the look and emotional appeal you wanted and at a price you can afford.
BUT: Whatever gemologists, traders, miners, jewelers or grading reports say, you are the only one authorized to judge beauty.