The Seng Jewelers Diamond Grading Explanation

Diamond Grading Explanatioin by Seng Jewelers


Seng Jewelers has been a proud member of the American Gem Society (AGS) for over 40 years.  The American Gem Society was established in 1934 by a select group of independent jewelers and Robert M. Shipley, found of the prestigious school of gemology, The Gemological Institute of America (GIA)


It was their vision to create an association dedicated to setting and maintaining the highest possible standards of business ethics and professionalism in the jewelry industry.  Today, American Gem Society members continue their dedication to ethics, knowledge and consumer protection.


The Diamond Grading information in the following paragraphs, will contain both the AGS and GIA grading Scales.  For additional information on these organizations, go to the GIA Website, or AGS Website.


Diamond's - The 4-C's

A diamond's valus is based on four factors:

Carat Weight  •  Cut (Proportions)  •  Color  •  Clarity


Carat Weight


Diamonds and other gemstones are measured by carat weight.  A carat is 1/5th of a gram, or 200 milligrams. Thekword  carat, comes from the Carob Bean, which weighs about 1/5th of a gram and was used as a measure in earlier times.  Stones that are lighter than a carat are measured in points.  There are 100 points in a carat.  One point equals .01 carat.


Caution:  Do not confuse the term carat with karat.  Karat is used to measure the fineness of gold (10K, 14K, 18K, etc.).Caratt weight refers to the size of a diamond.  The other three qualities describe the gem's quality.


One way to think about the 4-C's is to imagine a seesaw, with size (carat weight) on one end and quality (the other three C's) on the other.  Within a given price range, as size increases, quality must diminish.  If quality improves, size must diminish.  In other words, you can either spend more per carat for a higher quality stone, or you can get a larger stone and settle for lower overall quality.


Seng Jewelers has a large assortment of Diamonds in allqualities,rhowever,  we specialize in the Seng Firey Diamond™, which is the ultimate cut diamond.



Cut is the most important of the 4-C's.  It accounts for one half of the diamonds value.


Cut is the most misunderstood of the 4-C's.  It is often wrongly thought of as the shape of the stone.  This confusion exists because, of course, the raw material must be cut into a shape, and the confusion increases because shapes of diamonds are given names, like Round Brilliant Cut, Oval Cut, Emerald Cut, and so forth.

Cut, when speaking of one of the four qualities that give diamonds their value, actually refers to the geometric proportions of the gem.  The geometric proportions are important because a diamond is a prism that refracts, or bends light rays, breaking white light into the colors of the rainbow.  It is this refraction that unleashes the color spectrum in a way that gives a diamond its fire.  The optical proportions must be exact in order to achieve maximum brilliance.

In 1919, Marcel Tolkowsky, whose family name has become very prominent in the diamond cutting industry, released a set of proportions that since have become generally accepted as the way the best and most beautiful round brilliant diamonds shoud be cut.  This set of proportions, ratios, and angles illustrated above have become known to the world as the Tolkowsky, or the American Ideal Cut.


Light Refraction


Light enters from the top of the diamond, is funneled downward where it strikes facets at the bottom, then is refracted to the other facets of the stone again and again as it works its way back up, until it leaves the stone at the top and enters the eye of the observer.  Cutting proportions need to be extremely precise to achieve the best effect.

Proper Proportions


Many stone cuts lack proper proportion and refraction of light.  The cut may be too deep, allowing light to escape through the lower pavilion.  If the cut is too shallow, light rays leak away at the bottom.  Too much weight above or below the girdle affects the brilliance, and the stone lacks fire.

Waste of Rough Diamond


The illustration below left, shows two well proportioned gems cut from diamond rough.  Below right shows that the same piece of rough, could yield two larger stones (greater carat weight), but they would be poorly proportioned and much less attractive.

Waste is inevitable in cutting a gem from rough stone.  When cutting an ideally proportioned diamond, the average weight loss is about 50%.  In order to retain more wieght, stones may be poorly proportioned and may even lack symmetry, but because carat weight is what the unenlightened consumer values, it is how most stones are cut.


Cut and Durability


The cut also affects the durability of the stone.  The culet, the facet at the bottom of the diamond, is meant to flatten the point that would otherwise be vulnerable to chipping.  If the culet is not present, or is too small, the stone may chip more easily.


The girdle is where the setting holds the stone.  If the girdle is too thin, the diamond could easily chip in that area.  If the girdle is too thick, or unevenly cut, the stone will be problematic to set.

The cutting grade of a diamond is determined by analyzing the proportions and symmetry through the use of instruments and/or visual techniques.  Deductions are taken for departures from the ideal proportions for, table diameter, crown angle, girdle thickness, pavilion depth, culet size, centering, roundness, and finish.


Current knowledge and technology unknown to Tolkowsky have shown that a diamond with proportions slightly outside the Tolkowsky parameters can still perform extremely well and be beautiful.  It also has been discovered that some diamonds, which are cut to those strict standards, are not as efficient in how they manipulate light.  There can be a number of reasons for this.


AGS Cut Grading


The AGS Cut Grading System considers not only the proportions of a diamond, but also the craftsmanship of its overall symmetry and polish.  Most importantly, it uses technology to analyze the cut's impact on the diamond's overall performance.  The three categories that determine the overall  AGS Cut Grade are -- Light Performance, Proportions, and Finish.


The AGS Diamond Grading Standards evaluate three of the four value factors -- cut, color and clarity -- on its own 0-10 scale.  The scales begin at 0 (zero), the highest grade, and go down to 10, the lowest.  The three factors are expressed separately along with the fourth factor, the carat weight of the stone, for the final AGS Grade.

GIA Cut Grading


On January 1, 2006, the GIA Laboratory introduced new versions of the GIA Diamond Grading Report and Diamond Dossier®.  These new reports now provide a single, comprehensive cut grade for all standard round brilliant diamonds falling in the GIA D to Z color scale, and Flawless to I3 clarity scale.  Diamonds receive one of five cut grades from Excellent to Poor.

A step beyond Ideal


The Seng Fiery Diamond™

Cut from the heart by EightStar™


The Seng Firey Diamond™ goes beyond the traditional ideally proportioned diamond established in 1919.  A diamonds beauty depends on light return to produce brilliancy, fire and sparkle.  However, just because a diamond is well contoured and crafted, does not guarantee that it will reflect light to its fullest degree.


To ensure maximum light return, Seng Fiery Diamonds™ are cut for optical, rather than external symmetry.  This means that Seng Firey Diamonds™ display perfect light reflection patterns in a special instrument called the Symmetriscope.  The eight ray pattern is proof of perfect optical symmetry and full return of light.

The cutting process for a Seng Firey Diamond™ takes approximately 30 hours, compared to the five to seven hours for a traditional cut diamond.  Master cutters continually check the diamonds with the Symmetriscope throughout the entire cutting process to ensure that their labors lead to maximum brilliance, fire and sparkle.  This labor of love gives you the best of both worlds: external and optical symmetry.



The finest and most expensive diamonds are totally without color, like a drop of distilled water.  The rainbow hues a diamond flashes, derived from the light, is seperated into the colors of the spectrum.  Diamonds of lesser quality have a yellowish, brownish, or greyish cast.


Judging color is a job that can be performed only by experts with proper gem lab equipment.  To grade color, the gemologist places the diamond under white light that is constant and free of ultraviolet rays.  The stone is placed table down (that is, top down) and viewed through the pavilion.  It is more difficult to judge color if the stone is already mounted.


The tested diamond is compared to a set of five or more master stones whose colors have been accurately graded and certified by the Gemological Institute of America.

Master set of color grading stones

The GIA color grading scale ranges from D through Z.  (A,B & C are not used to avoid confusion with other grading systems that use only those three letters.)


GIA D-F / AGS 0-1.0

These diamonds are colorless.


GIA G-J / AGS 1.5-3.0

These diamonds are considered near colorless.  Small stones appear colorless when mounted, but large stones appear tinted to the trained eye.


GIA K-M / AGS 3.5-4.5

Slight traces of color are apparent in mounted stones only to the trained eye.


GIA N-Z / AGS 5.0-11

Mounted stones will display a yellowish tint, even to the untrained eye.


The gemologist assigns the stones a specific letter grade.  All other things being equal, the lower the color grade, the lower the stone's value.


"Fancy Colored" Diamonds

Diamonds are found in almost every color.  The Hope Diamond is blue.  Other diamond are fancy yellow, pink, rose, and green.


To avoid confusion, diamonds of intense color are referred to as "fancies".  Diamonds of intense color are rare in nature.  Those of exceptional quality are very expensive and considered collector's items.


The majority of "fancy colored" diamond are not natural in color, but are color enhanced by irradiation or other means.  These treatments are done to diamonds of poor color that would have low value in their natural state.



Clarity is the term used to describe the internal quality of a gem.  A trained expert examines the diamond for inclusions, cracks, spots, clouds, or any other blemish or imperfection of any sort.  Usually blemishes cannot be seen with the unaided eye, but can be detected under 10 power magnification.  Refer to the photo below.

Clarity Grading Levels


The GIA and AGS recognize 10 grading levels for clarity.  Other qualities being equal, the lower the clarity grade, the less valuable the stone.

Clarity Grade Definitions


IF - GIA = Internally Flawless / AGS - 0

Free of flaws, cracks, spots, clouds, or any other blemish or imperfection of any sort when examined under proper light by a trained eye with the aid of a diamond eye loupe, or other magnifier with a magnification of 10 power.


VVS1 - GIA = Very, Very Slightly Included / AGS - 1

The gem may have a very tiny pinpoint of included material, cloud, polishing line, or faint knot line, but only one of these blemishes and it is very faint to a trained eye under 10X magnification.


VVS2 - GIA = Very, Very Slightly Included / AGS - 2

Inclusions may be the same as for VVS1, but slightly larger and more numerous.  They are still very hard to see.


VS1 - GIA = Very Slightly Included / AGS - 3

The gem may have minute internal cleavage or fracture feathers near the girdle, or it may have any of the above blemishes, only slightly larger, but still fairly difficult to detect under 10X Magnification.


VS2 - GIA = Very Slightly Included / AGS - 4

Any of the above inclusions that are more easily visible under a 10X loupe.  An included crystal or other small blemish may be seen through the crown.


SI1 - GIA = Slightly Included / AGS - 5

The diamond has a cleavage fracture, or any blemish or combination of blemishes not visible to the unaided eye, but easily seen under 10X magnification.  A small dark spot in the center of the stone or a larger white flaw toward the edge would be graded SI.


SI2 - GIA = Slightly Included / AGS - 6

The diamond has slightly larger inclusions than an SI1, but they still are not visible to the unaided eye when the stone is fact up.


Note: There is no such thing as a GIA grade of SI3.  Such a stone is really just an I1 grade.


I1 - GIA = Included / AGS - 7

Inclusions are just visible to the eye without a loupe.


I2 - GIA = Included / AGS - 8

Inclusions are easily seen with the unaided eye.


I3 - GIA = Included / AGS - 9 & 10

This grade denotes a badly included diamond with cleavages, fractures, large clouds, and dark spots large enough to block light passage and destroy brilliance.  This grade of diamond would be inappropriate for jewelry.


Excessive inclusions or fractures not only make the diamond unattractive, but may weaken its strength so it is more likely to crack.  However, flawless diamonds are extremely rare in nature.  One expert points out that no two diamonds are alike, and the inclusions in a particular stone are like its fingerprint, identifying it among all others.

Seng Jewelers, Downtown Louisville, KY

Hours, 10:00 - 5:15, Tuesday - Saturday

Phone: 502-585-5109



One Riverfront Plaza  • Louisville, KY 40202



Seng Jewelers is Louisville's number one source for Ultimate Cut Diamond Engagement Rings.  Choose the finest, choose the Seng Firey Diamond™

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