Reading a Diamond Certificate for a Round Shape: Some helpful hints
For even a well-educated consumer, much of the information on a diamond grading report can look like gibberish. In the
interest of helping you find the diamond that is best suited to your expectations, we offer the explanations and opinions
below. There are always exceptions to the rule, but this should serve as a concise guide in most situations. If you have questions about a certificate, we encourage you to call our head diamond buyer, Andy Marcum at 800-630-2686
1. Evaluating the Cut dimensions of a diamond
If you know how to read them correctly, the dimensions are the key to how much brilliance and fire your diamond will display.
Every report lists the table percentage and total depth of a diamond. One popular formula
among diamond experts is the
60/60 rule. If both the table percentage and
total depth of a diamond equals near 60% of the width, you probably have a good value on your hands. The 60% table makes the
diamond appear larger when viewed from the top down and the depth is sufficient to
create a good return of light. However, this is an estimation. There is not really enough
information here to properly evaluate the brilliance of a diamond.
GIA reports (which are very reliable), and some others do not provide additional specifications
on diamond dimensions. They do, however, list some important information in a text format as
shown at left. The Symmetry rating is a good indication of Cut. In the Proportions listing, you
can also estimate that a table under 50% or over 65% will sacrifice the the brilliance and beauty
of a stone. The smaller table will result in a deep cut that usually appears dark in the center,
while the larger table will result in a shallow cut, that usually appears milky and dull.
To really evaluate the brilliance, fire and scintillation of a diamond, you need a report that
lists all of the dimensions of a diamond, which many reports contain.
We don’t want to over complicate matters, but in a grading report that lists all of the angles of a
diamond cut the most crucial factor is the pavilion depth. The optimum pavilion depth for the
most beautiful. brilliant diamond is 43.1%. A depth near this will create a dramatic shower of
brilliance, scintillation and fire to the eye of the viewer. This falls within the AGS (0) or Ideal Cut
standard, and these diamonds command a premium price in the marketplace. If you find an
Excellent Cut near this best pavilion depth, you probably have a good bargain on your hands.
The three important dimensions that determine brilliance are the Pavilion Depth, the Table % Diameter and Total Depth % Diameter. AGS has established strict tolerances
for diamond cut grades. The three examples below illustrate the brightest diamond (0) Ideal,
a (2) Very Good diamond and a (6) Fair diamond, which would be dull and lifeless in comparison.
AGS (0) Ideal Tolerances
Pavilion Depth: 42.2%-43.8%
Table Diameter: 52.4%-57.5%
Total Depth Diameter: 59.8%-62.4% |
According to traditional guidelines, this diamond will exhibit the best return of brilliance, fire and
scintillation. |
AGS (2) Very Good Tolerances
Pavilion Depth: 41.7%-42.1% or 44.4%-44.8%
Table Diameter: 59.6%-61.5%
Total Depth % Diameter: 55%-63.9% |
Some experts feel that a 60% table diameter
combined with a 60% depth provides better
value for the cost, an excellent return of light
and a larger looking diamond. However, the
optimal pavilion depth is still 43.1% |
AGS (6) Fair Tolerances
Pavilion Depth: 45.9%-46.3%
Table % Diameter: 67.6%-69.5%
Total Depth % Diameter: below 53% |
Judged by any standards this is a dull and
lifeless diamond. The table should not exceed
63% and the pavilion depth will leak a substantial amount of light. |
2. Which Gem Lab issued the diamond grading report?
This is a crucial and often overlooked factor in the evaluation of your diamond. We’ve been buying diamonds for over 40 years
and can say without reservation that some labs are very strict in their Color and Clarity grades and some labs are soft in their
grades. When it comes to Cut, the numbers speak for themselves. However, Color and Clarity are subjective to a gemologist’s
opinion. A single bump in either Color or Clarity grade will increase the market value of a 1 carat diamond by about 10% and a
2 carat diamond by about 20%.
If you view a diamond in our inventory that is priced substantially less than another diamond with similar Weight, Cut, Clarity and
Color standards, take note of the lab that issued the grading report. The most reliable and strict grading reports are issued by
AGS and GIA. This does not mean that buying a diamond with a report from another lab is a bad deal. You should simply be
aware that the Color or Clarity may be a grade lower than stated, and evaluate the price accordingly .
If you find what appears to be an extraordinary value on a stone evaluated by EGL, IGI, HRD, or AIG, we invite you to call
our diamond buyer, Andy Marcum at 800-630-2686. He will evaluate the stone personally and issue an informal opinion, at no cost, that is probably
more representative of the true value of the diamond you have in mind. It may be a great bargain, even if the gemlab certificate
overstates Color or Clarity.
There are several articles in the trade press that document the differences in grade ratings from various labs. You can read more here,
or ask any trusted local jeweler about their opinion. The industry consensus is broad and in general agreement about the reliability of
grading from different gem labs.
3. Evaluating the Clarity grade of a diamond
The most important tip to evaluating the Clarity grade of a diamond is to examine the "plot" or map of the stone. The plot is made by
an individual gemologist usually looking at the diamond under a 10x magnification. Not all certificates include a plot, and you should be
wary of the that do not. When you read these plots, keep in mind that the flaws in a diamond rated SI2 or greater are "eye clean".
meaning that they can not be seen with the naked eye. Higher grades are more rare and expensive, but you will pay a premium price
for quality that is not discernible without the aid of a loupe or microscope.
Here is a plot for an SI1 Clarity diamond that looks like a pretty
good value. First, notice that most of the inclusions are toward
the outer edge of the diamond. Inclusions toward the center of
the diamond are more noticeable, and they reflect off more facets
of the diamond, multiplying their appearance. Also, sometimes
inclusions on the outer edge can be hidden under prongs when
the stone is set, so they won’t be seen.
The top view of this plot indicates that there are several Needle
inclusions near the outer edge, a small Cloud inclusion in
the upper left area and a pinpoint Crystal inside the upper
area of the table. However, none of these inclusions will
seriously degrade the beauty of this diamond.
The actual diamond images shown below illustrate the types of inclusions and how they will
appear under 30X magnification
in a diamond. These inclusions
would not be visible to the
naked eye.
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Feather |
Crystal |
Cloud |
Needle |
4. Evaluating the Color grade of a diamond
Among all the qualities of a diamond, Color is the most subjective, because it has no effect on the brilliance, fire or scintillation.
You may be surprised to learn that diamond Color is graded in loose stones viewed upside down
and the nuances of color are
very difficult for even an expert to distinguish. From the upside
down angle, most people cannot tell the difference between a
D and F Color. When a diamond is mounted and viewed from
the top down, it is difficult to see the difference between even a
D to H Color. However, each grade increase in Color will cost an
additional 10% in a 1 carat diamond and about 20% in a 2 carat
diamond. This is why we often advise customers who are looking
for the best value to choose a G-H Color diamond.
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