|Quantifying the Quality of a Gem
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|Author:||Barbra Voltaire, FGG [ Fri Aug 24, 2018 2:38 pm ]|
|Post subject:||Quantifying the Quality of a Gem|
May I take this opportunity to paraphrase of Richard Wise's classic, "Secrets of the Gem Trade"
Richard emphasizes that a dealer who claims "beauty is in the eye of the beholder" is giving their clients tainted misinformation. The quality of a gem can indeed be quantified.
He sites AGL's Color Scan grading system as an excellent example. I agree:
The American Gemological Laboratories (AGL) Colored Stone Grading Report
How to Determine the Quality of a Colored Gemstone
In 1977, American Gemological Laboratories (AGL) became the first laboratory in the world to quantify colored stones in a linear, comparative manner. The organization and its President, C. R. Beesley, are internationally recognized for their expertise in colored stone research and analysis. They have enjoyed a position of leadership in gemstone documentation and have extensive experience in consumer protection programs.
The grading system which they devised is comprehensive and requires some study to realize a workable understanding of the process. However, once you grasp the basic principles, you will wonder how anyone could buy a gemstone without using this kind of vocabulary. The following "walk through" of a 3.11 Burma sapphire grading report will help you understand the various components of an AGL laboratory report.
The lab assigns individual, consecutive numbers for each grading report issued.
A color digital image of the stone is an important part of the grading report. This image assists in verifying the gemstone matches the grading report. Please note the image is for representational purposes and is not necessarily the actual size or color.
In this sample, the AGL has determined the stone is a natural sapphire.
Shape and Cut
The stone is an oval cut in this sample.
Colored stones are weighed with an extremely accurate scale. In the sample, the sapphire weighs 3.119 carats.
The measurement of the gemstone in millimeters.
Color Rating Scale (AGL)
3....4 Very good
Since there are so many optical/physical principles involved in the reflection and re-reflection of light through a gemstone, any particular gem can exhibit a wide range of colors. The color grade is based upon the purity of primary color. For example, the greener the emerald, the redder the ruby, or the bluer the sapphire, the lower the numerical grade on an AGL scale. The reason the 3.11 sapphire is a 3.5 in this sample is the fact the stone has 70% blue as its primary color. It is that simple. The less primary color, the higher the numerical color grade. For example, a 65% primary color would probably be graded a 4.5 color.
In establishing a base for international quality reference standards, AGL had to be sure that all stones from the best to the worst could be accommodated by the system. Therefore, the top of the line has become almost a theoretical standard of excellence. Please do not ask for 1.00 to 2.00 color grades. Because of the AGL's stringent grading practices, a 3.5 color is usually the best color grade any gem will receive. A 3.5 colored stone on an AGL Grading Report is nearly synonymous with a D-color diamond on a GIA Diamond Grading Report. On occasion, because of our international contacts, we may find exceptional examples that will receive even better color grades.
Tone Scale (AGL)
0..5..15..20 Very Light
90..95..100 Dark-Very Dark
In the sample above the sapphire's tone is 75. Basically, tone describes the lightness or darkness of a gemstone. Color considered alone without taking into account the tone will lead to errors in interpretation. If a stone is too light in tone, the color will not be rich enough. Conversely, if a stone is too dark, it will sacrifice transparency and brilliancy.
Clarity Scale (AGL)
FI Free of Inclusions
LI1...LI2 Lightly Included
MI1...MI2 Moderately Included
HI1...HI2 Heavily Included
E1...E2...E3 Excessively Included
Clarity is defined as the degree to which the stone is free of inclusions. In the sample the clarity is MI1. Here is a good rule of thumb to remember: You will probably not see inclusions in a Lightly Included (LI) gem, you may see some inclusions in a Moderately Included (MI) gem, and you will probably see inclusions in a Heavily Included (HI) gem. Excessively Included (E) gems have durability problems and should be avoided.
The depth of the gemstone.
Cutting/Finish Scale (AGL)
2...3 Very Good
The cutting of a gemstone numerically describes the overall appearance of a gem. The finish grade refers to the polish of a gem. In the sample the cutting and finish are both 3 (Very Good). These are very difficult numbers to obtain for a colored gemstone. Unlike diamonds which are cut according to strict mathematical parameters, most colored gemstones are cut for weight retention.
Brilliancy is the amount of flash the gem returns to your eyes. Most diamonds are 100% brilliant. Brilliancy in the sample is 80%. An average brilliancy of 50% means half of the stone returns flash.
The comment section is critical because it generally designates the country of origin of the gemstone as well as specifying what, if any, treatment(s) the gem has been subjected to. In the sample, the 3.11 is classified as Burma without heat enhancement.
Some gems may receive an Estimated Commercial Acceptability (ECA) comment. (Not shown in sample.) This is usually used as a split grade. For example, a gemstone with a 4 color and an ECA of 3.5 is priced as a 3.75 color grade.
Total Quality Integration Rating
This comment integrates the total visual impact of the gem into a verbal description and may or may not have an effect on the price of the gem. In the sample it is Excellent. Look at the Color Rating (AGL). Excellent translates into 1.5. The TQIR can increase or decrease the final grade only 1/2 of a grade. Therefore, in this sample, a TQIR of 1.5 raises the 3.5 color grade to a 3. If the TQIR is lower than the color grade, it means the stone may have a problem, and the TQIR can actually lower the final grade of the stone 1/2 grade.
Carat Weight: 1 carat or larger
Color: Good (6) or better
Tone: Ideal tone for ruby, sapphire and emerald ranges from medium to dark (65-85). The only exception is no treatment emeralds can be lighter in tone. Ideal tone for other gem types varies from light to medium (20-65).
Clarity: Moderately Included Two (MI2) or better
Average Brilliancy: 50% or more
Cutting/Finish: Good (6) or better
For more information, visit AGL's web site.
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