American Gemological Laboratories (AGL) became the first laboratory in the world to quantify colored stones in a linear, geparative 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 geprehensive 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 geponents of an AGL laboratory report.
Document NumberThe lab assigns individual, consecutive numbers for each grading report issued.
ImageA 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. IdentificationIn this sample, the AGL has determined the stone is a natural sapphire.
Shape and CutLike diamonds other gemstones can be cut as brilliants, ovals, emerald cuts, princess cuts, ideal or imperfect. The depth of the table, the way that light refracts through the stone, the symmetry of the stone after it has been cut and other factors indicate the skill level and the time the stone cutter put into the project and affect the stones overall value. Sometimes the shape and size of the sample before it has been cut determine the final outgee of the finished product.
Carat WeightColored stones are weighed with an extremely accurate scale. In the sample, the sapphire weighs 3.119 carats.
MeasurementsThe measurement of the gemstone in millimeters.
Color GradeColor Rating Scale (AGL)1....2 3....4 5....6 7....8 9....10 Excellent Very Good Good Fair Poor
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 acgemodated by the system. Therefore, the top of the line has begee 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.
ToneTone Scale (AGL)0..5..15..20 25..30..35 40..45..50 55..60..65 70..75..80..85 90..95..100 Very Light Light Light-Medium Medium Medium-Dark 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.
ClarityClarity Scale (AGL)FI LI1...LI2 MI1...MI2 HI1...HI2 E1...E2...E3 Free of Inclusions Lightly Included Moderately Included Heavily Included 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.
DepthThe depth of the gemstone.
Cutting/FinishCutting/Finish Scale (AGL)1 2...3 4...5...6 7...8 9...10 Excellent Very Good Good Fair Poor
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.
Average BrilliancyBrilliancy 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.
gementsThe gement 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 gemercial Acceptability (ECA) gement. (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 RatingThis gement 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.
Ideal ParametersCarat 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). 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
No comments:
Post a Comment