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Diamond Grading Systems

For round diamonds we use The Gemological Institute of America’s universally accepted “Make” classifications that define the range of empirical measurements for round diamonds and that also governs the interpretive aspects (symmetry and polish) of cut. GIA divides round diamonds into four make classes, Make Class One and Make Class Two (preferred), Make Class Three and Make Class Four (average and poor).

AGS CertificateFor round Ideal Cut diamonds we use the American Gem Society’s grading system that rates each proportion and finish category from 0 (preferred) to 10 (poor). An AGS Triple Zero, the finest round diamond in the world, has Ideal proportions, polish, and symmetry.

For “fancy” shape diamonds, all standard shapes other than round, it is my experience that the grading system developed by Mr. David Atlas and his Accredited Gem Appraisers provides the most realistic assessment of grading variants to the actual appearance of the diamond. AGA grades fancy cut diamonds as fine, average and below average.

DEPTH

Grading For The Amount of Refracted Light (Fire) Returned To The Eye From Within The Diamond:

One kind of light we see viewing a diamond from the crown (top view) is light viewer’s eye is, primarily, a function of the depth of the diamonds pavilion and the resulting degree of the pavilion angle. By using grading systems from the three aforementioned professional organizations we are able to judge the amount of refracted light that is returned to the viewers eye from within the various shapes of diamonds.

For Round Diamonds GIA grades the pavilion depth with four Make Classes: align

  • Make Class One requires a pavilion depth of 43%
  • Make Class Two requires a pavilion depth of 42 to 44%
  • Make Class Three has a pavilion depth of 41 to 46%
  • Make Class Four has a pavilion depth less than 41% or more than 46%

For Round Ideal Cut Diamonds AGS considers both the pavilion depth and the pavilion angle.

Ideal Cut requires a pavilion depth between 42.2% and 43.8% that has entered the diamond through the top, reflected off the interior pavilion (bottom) facets and returned to the eye. This light is refracted (bent) by the molecular density of the diamond whereby white light is separated into its component of rainbow colors. The amount of refracted light that is returned to the eye.

  • Ideal cut requires a pavilion angle between 40.5 degrees and 41.5 degrees

For Pear, Heart, Oval and Marquis Shape Diamonds AGA grades the Total Depth Percentage as follows:

  • Fine Grade A requires a total depth between 59.0% and 63%
  • Fine Grade B requires a total depth between 58% and 65.4%
  • Average Grade A indicates a total depth between 56% and 68.5%
  • Average Grade B indicates a total depth between 46% and 71%
  • Below Average Grade A indicates depth between 43% and 73%
  • Below Average Grade B indicates total depth < 43% or > 73%

For Emerald and Radiant Shape Diamonds AGA grades the Total Depth Percentage as follows:

  • Fine Grade A requires a total depth between 60% and 65%
  • Fine Grade B requires a total depth between 58% and 69%
  • Average Grade A indicates a total depth between 57% and 74%
  • Average Grade B indicates a total depth between 56% and 78%
  • Below Average Grade A indicates total depth between 53% and 80%
  • Below Average Grade B indicates total depth lower than 53% and higher than 80%

For Princess Shape Diamonds AGA grades the Total Depth Percentage al follows:

  • Fine Grade A requires a total depth between 64% and 75%
  • Fine Grade B requires a total depth between 58% and 80%
  • Average Grade A indicates a total depth between 57% and 83%
  • Average Grade B indicates a total depth between 56% and 84%
  • Below Average Grade A indicates total depth between 53% and 85%
  • Below Average Grade B indicates a total depth <53% or >85%

When a diamond is cut too deep, increasing the pavilion angle, light entering the top of the diamond leaks out the sides rather than being returned to the viewer’s eye and the diamond becomes dark and obscure. We call such a diamond a nailhead because the center top view is dark. Additional carat weight is used and wasted in such extra depth with the result being a smaller diameter than would be characteristic of a diamond more optimally cut.

When a diamond is cut to shallow with a low pavilion angle the diamond becomes lifeless from the top view and it is called a wafer. With both nailheads and wafers spotlights can compensate for their dark or dull, lifeless appearance by causing excessive flash from the exterior crown facets. Buyer Beware of jewelers demonstrating diamonds under spotlights as it is the first indication of the jeweler’s intent to fool you into a false impression of the beauty of the diamond and you should take demonstrations under spotlights as a red flag to indicate other deceptive and misleading practices are quite possible.

Bow-ties are a dark, obscure phenomena occurring in the center of elongated diamonds that have a pointed culet causing a dark obscurity that travels up and down the length of a diamond as it is tipped from one end to another. The effect if this major cutting deficiency is a dark, lifeless and unattractive center. The cause is this phenomena is the increased pavilion angle at the culet center, the deepest part of the diamond, an angle so deep as to cause refracted light in the center of the diamond to leak out the sides of the stone rather than being returned through the crown (top) and to the viewers eye. Although the bow ties in oval, pear and marquis shape diamonds can never be completely eliminated by virtue of their cutting styles, the bow-tie can be kept to a minimum. For the oval, pear and marquis diamond it is most important to insist on the Fine grade A depth percentage of between 59% and 63% to eliminate as much of the bow tie effect as is possible for those diamond shapes.

TABLE

Grading For The Mix Of Flash From The Exterior Facets And Fire From Within The Diamond

A second kind of light we see in viewing a diamond from the crown (top) view is the “flash” of environmental light that reflects from the adamantine (hard) exterior crown facets of a diamond. The Blend of flash from the external top facets and dispersion (fire) from within the diamond is graded by the table percentage. In general, the larger the table diameter the more that flash overpowers and obscures the refracted light (fire) from within the diamond. Then, when the crown angle varies from optimal, the blend (balance) of flash and fire is altered causing an unequally balanced display of two of the components of light we see in a diamond, flash and fire (body color is the third component of light). By using the grading systems of three professional groups, Gemological Institute of America, The American Gem Society and The Accredited Gem Appraisers we are able to grade the mix of refracted light that is returned to the viewers eye from within the various shapes of diamonds and the flash of exterior light reflecting from the crown facets. In referring to the Gemological Institute of America diamond grading system or the Accredited Gem Appraisers fancy shape diamond grading system we will use only the grading information contained on a Gemological Institute of America Diamond Grading Report, a certificate that does not state the crown angle or height.

For Round Diamonds GIA grades the Table Diameter with four Make Classes:

  • Make Class One requires a Table Diameter between 53% and 60%
  • Make Class Two requires a Table Diameter between 61% and 64%
  • Make Class Three indicates a Table Diameter between 65% and 70%
  • Make Class Four indicates a Table Diameter in excess of 70%

For Round Ideal Cut Diamonds AGS considers both the table diameter and the crown angle:align

  • Ideal Cut requires a table diameter between 52.4% and 57.5%
  • Ideal Cut requires a crown angle between 33.7 degrees and 35.8 degrees

For Pear, Heart, Oval and Marquis Shape Diamonds AGA requires a table percentage as follows:

  • Fine Grade A requires a table percentage between 53% and 62%
  • Fine Grade B requires a table percentage between 52% and 64%
  • Average Grade A indicates a table percentage between 51% and 67%
  • Average Grade B indicates a table percentage between 50% and 70%
  • Below Average Grade A indicates a table percentage between 49% and 72%
  • Below Average Grade B indicates a table percentage < 49% or >72%

For Emerald Shape and Radiant Diamonds AGA requires table percentages as follows:

  • Fine Grade A requires a table percentage between 59% and 65%
  • Fine Grade B requires a table percentage between 58% and 69%
  • Average Grade A indicates a table percentage between 56% and 74%
  • Average Grade B indicates a table percentage between 53% and 76%
  • Below Average Grade A indicates a table percentage between 50% and 78%
  • Below Average Grade B indicates a table percentage < 50% or >78%

For Princess Shape Diamonds AGA requires table percentages as follows:

  • Fine Grade A requires a table percentage between 59% and 72%
  • Fine Grade B requires a table percentage between 58% and 77%
  • Average Grade A indicates a table percentage between 56% and 82%
  • Average Grade B indicates a table percentage between 53% and 85%
  • Below Average Grade A indicates a table percentage between 50% and 88%
  • Below Average Grade B indicates a table percentage < 50% or > 88%

Diamonds that are lifeless and obscure because of poor table percentages and/or poor depth percentages can be artificially enhanced by the use of multi-directional spotlights and you should take the use of spotlights for demonstrating diamonds as the jewelers attempt to cheat the actual appearance of the diamond – Buyer Beware. Diamond grading environments are actually less expensive to maintain than are the spotlight showrooms of too many jewelry stores.

More on Grading Systems

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