Gemological
Institute of America (G.I.A.) and American Gem Society (A.G.S.)
diamond reports are the most accurate and reliable available.
I rarely use other reports.
G.I.A.
Reports-- Most of the information you need to evaluate
the quality of a round brilliant cut diamond is contained in
a G.I.A. Report. However, two pieces of information which
are critical in determining whether a diamond is well-cut are
missing from the G.I.A. Report-- i.e., the diamond's crown angle
and pavilion depth/angle. Unless you are a trained gemologist
with a great deal of experience, the crown angle and pavilion
depth can only be obtained from a "Sarin Report" which is produced
by a technically sophisticated computerized machine called a
"Sarin Brilliant-Eye" (or a similar device called a "Megascope").
This machine records all of the angles and measurements of a
diamond. A Sarin Report is available for all diamonds sold by
Dimonz. Please read the segment of this website on Cut under the "Four C's" section of this website, before reading
this section.
A.G.S.
Reports -- All of the information necessary to evaluate
the quality of a round brilliant cut diamond is contained in
an A.G.S. report, including the crown angle and pavilion depth/angle.
The
information presented below deals only with "round brilliant"cut
diamonds. The author believes that diamond grading reports do
not contain sufficient information for customers to confidently
purchase other shapes via the internet without professional
assistance (i.e., pears, ovals, marquises, emerald-cut, princesses,
radiants, heart-shaped, etc.).
Each field on G.I.A.'s diamond grading report is explained below
(Note: the same information is contained on A.G.S. reports
in a slightly different format).
Shape
and Cutting Style
Measurements
Weight
Depth
Table
Girdle
Culet
Polish
Symmetry
Clarity Grade
Color
Fluorescence
Comments
-
As stated, this field describes the shape and cutting style
of the diamond (e.g., "round brilliant").
- Example--
6.45 - 6.52MM (min. and max. width)X 3.91 MM (depth).
To
avoid buying a stone that is either too shallow or too deep,
take an average of the first two measurements, which represent
the maximum and minimum width measurements of the diamond (in
the example above, 6.48 mm is the average diameter- i.e., 6.45mm+6.52mm
divided by 2). The average should be approximately as follows:
| |
Ave. |
Approx.
Acceptable Range |
| .50 carat |
5.2 mm |
(5.1mm-5.3mm) |
| .75 carat |
5.9 mm |
(5.75mm - 6.0mm) |
| .85 carat |
6.2mm |
(6.05mm-6.3mm) |
| 1.00 carat |
6.5mm |
(6.4mm-6.6mm) |
| 1.25 carats |
7.0mm |
(6.85mm-7.1mm) |
| 1.50 carats |
7.4mm |
(7.25m-7.5mm) |
| 1.75 carats |
7.8mm |
(7.6mm-7.95mm) |
| 2.00 carats |
8.2mm |
(8.0mm-8.35mm) |
| 2.25 carats |
8.6mm |
(8.45mm-8.75mm) |
| 2.50 carats |
9.0mm |
(8.75mm-9.2mm) |
If
you have to interpolate between sizes, there is a direct linear
relationship. For example, a diamond weighing 1.12 or 1.13 carats
should have an Average Girdle Diameter of approximately 6.75
mm (i.e., halfway between 6.5 mm for a 1.00 carat and 7.0 mm
for 1.25 carat).
- Weight of the
diamond in carats. A carat is equal to 1/5 of a gram.
- On a G.I.A.
Certificate, this figure represents the depth of the diamond
expressed as a percentage of it's width. To get the "depth percentage",
you take the depth of the stone in millimeters (the last figure
under measurements on a G.I.A. report) and divide by the diameter
(i.e., the Average Girdle Diameter). Generally
speaking, the Depth Percentage should be between 59% and
63.0%. Shallow stones with a depth percentage of
less than 58% are called "spready makes" in the gem trade. They
appear to be larger than normally cut stones of the same weight
because they are wider but they tend to be less brilliant because
the pavilion angle is too shallow and light leakage results
(Diagram A, far right figure). Stones with a depth of over 64%
are termed "heavy" or "deep makes" (Diagram A, middle figure).
These stones will be less brilliant than normally cut stones
and may appear to be dark because much of the light leaks out
of the bottom or pavilion of the stone because the pavilion
angle is too steep. Also called "Nailheads" (because they look
dark), these stones also appear to be smaller than normally
cut stones of the same weight because much of the weight is
in the depth of the stone.
Although
the Total Depth Percentage may give a rough idea as to whether
the diamond is too shallow or too deep to play well with light
when the measurements exceed certain parameters (e.g., if it
is under 58% or over 64%), it is too imprecise in determining
the optical efficiency of a stones which fall within the recommended
range. For example, any diamond with a 59.5%-63.5% total depth
percentage could be "ideal cut" with a perfect 43% pavilion
depth, a 40.8 degree pavilion angle and a 34.5 degree crown
angle or it could just as easily be abominably cut, with a 46%
pavilion depth, a 43 degree pavilion angle and a 29 degree crown
angle. For that matter, it could be the reverse, with a high,
steep crown and shallow pavilion.
Quite
frankly, in and of itself, the total depth percentage of a diamond
is usually of negligible value. You need the pavilion depth/angle
to determine whether the diamond will leak light because the
pavilion is too deep or too shallow.
Diagrams
of play of light
DIAGRAM
A
- This figure represents the table percentage which, like the
depth percentage, is one of the most important measurements
of the diamond. This is the relative size of the table (the
large flat area across the top of the diamond), expressed as
a percentage of the width of the stone. For example, if a diamond
with a 6.50 millimeter diameter (i.e., the Average Girdle Diameter)
has a table measuring 3.90 millimeters, the table percentage
is 60% (3.9 mm divided by 6.5 mm = 60%). Generally
speaking, the smaller the table %, the better the play of light
in the diamond.
There are two types of brilliance which come from a diamond:
life and fire. The "life" of the diamond is the white light
that is reflected from the table. The "fire" of the diamond
is the multicolored, scintillating light that comes from the
diamond. The fire comes from the "crown facets" that surround
the table and is regarded as being more beautiful than the "life" or white light.
Smaller
tables are desirable because, diamonds with small tables generally
have more fire and are more aesthetically pleasing than diamonds
with larger tables. However, diamond cutters frequently cut
large tables to save weight from the rough.
DIAGRAM B
53%-57.5%
- Ideal - If all the other factors on the G.I.A.
And Sarin Reports are acceptable (e.g. a crown angle of 33.7
to 35.8 degrees; a pavilion depth of 42.2%-43.8%; Very Good
or Excellent polish and symmetry; and girdle and culet measurements
as defined, below) you will generally pay a premium of 10% to
15% for stones with table percentages in this range, which produce
exceptional "fire".
57.6-60%
- Superior - If all the other measurements of the
diamond are within acceptable parameters, diamonds with tables
in this range demonstrate excellent brilliance and fire.
61%-62%
- Good - Typical of diamonds having an "international
fine trade cut." If all the other measurements of the diamond
are within acceptable parameters, diamonds with tables in this
range demonstrate a reasonable amount of brilliance and fire.
Dimonz generally does not sell round diamonds with a table larger
than 62%.
63%-64%
- Marginal - Diamonds with a 64% table generally
display a fair amount of fire, and typically sell about 5-10%
less than those with a table from 61% to 62%, assuming all other
measurements are acceptable.
65%
and above - Don't Buy; too little fire
6.
GIRDLE- This factor measures
the thickness of the "girdle" or the outside circumference of
the diamond. There are typically two measurements given. The
first measurement reflects the minimum thickness and the second
reflects the maximum thickness for the girdle of the diamond.
If the girdle is the same thickness all the way round, there
will be only one measurement given. The measurements are as
follows: Extremely Thin, Very Thin, Thin, Medium, Slightly Thick,
Thick, Very thick, Extremely Thick.
DIAGRAM C
Do
not buy a diamond that has "extremely thin" as a girdle measurement
-- it may be prone to chipping along the girdle. Don't buy a
diamond that has "very thick" or "extremely thick" as a girdle
measurement-- the cutter has added unnecessary weight to the
diamond which you will pay for! Also don't buy or those have
a single measurement of "thick." Again, the cutter has added
unnecessary weight to the diamond.
Generally,
the following girdle thicknesses are acceptable: Thin; Thin
to Medium; Thin to Slightly Thick; Thin to Thick; Medium; Medium
to Slightly Thick; Slightly thick; Medium to Thick.
Diamonds with Very Thin as one of the measurements may also
be acceptable.
The culet is a very small facet on the pointed end of the diamond.
The following culet sizes are acceptable:
none, very small, small, or medium. Do not buy a
diamond with a large or very large culet. The latter are undesirable
because a large culet may be eye visible through the top of
the stone, appearing as a dark spot or small black hole in the
center of the diamond.
DIAGRAM D
FINISH (I.E., POLISH AND SYMMETRY)-- Each
field under finish is discussed below
- This element
measures the quality of the finish on the surface of the diamond,
e.g., the presence of abrasions, polishing lines and other surface
characteristics that were made when the diamond was cut. There
are five grades- i.e., Poor, Fair, Good, Very Good, Excellent.
Any diamond you consider purchasing
should have a polish grade of "Good" or better. Premium cut
stones (i.e., G.I.A. Class I and A.G.S. Ideal or "000" cut grades;
see discussion under Cut) should
have a polish grade of Very Good or Excellent.
- This element
measures the shape and overall cutting detail of the facets.
There are five grades- i.e., Poor, Fair, Good, Very Good, Excellent.
Any diamond you consider purchasing
should have a symmetry grade of "Good" or better. Premium cut
stones (i.e., G.I.A. Class I and A.G.S. Ideal or "000" cut grades;
see discussion under Cut) should
have a symmetry grade of Very Good or Excellent.
- Please
see discussion under "Clarity" on this web-site. Generally,
I recommend VS1, VS2 or SI1 clarity as representing the best
value.
- Please see discussion
under "Color" on this
web-site. Generally, I recommend G,
H or I color as representing the best value.
- G.I.A.
assigns one of five levels for this element: None (i.e., no
fluorescence), faint, medium, strong, very strong. Generally
speaking, ratings of "none", "faint blue" or "medium blue" fluorescence
have little or no impact on the value or aesthetic appeal or
value of a diamond in the F to I color and VS1 to SI2 clarity
range. In most cases, strong or very strong blue fluorescence
does not adversely affect the appearance of a diamond. However,
a small percentage of strongly fluorescent diamonds appear milky
when exposed to strong ultraviolet light. It should also be
noted that diamonds with strong or very strong blue fluorescence
in the D through H color range in VS and better clarities, generally
discount 3%-10% from those with faint blue or without fluorescence.
When buying diamonds of in the I,J,K color range, you may pay
slight premiums for stones with blue fluorescence because the
blue negates the yellow tint in these diamonds and makes them
appear whiter than they actually are. Frequently, a good way
to save money is to buy a diamond with a slightly lower color
that appears much whiter when mounted due to it's fluorescence.
The stronger the fluorescence, the whiter the stone looks.
What
is Fluorescence? Diamonds are composed of carbon. They develop
approximately 100 miles deep in the earth from a form of carbon
called graphite (the same as in pencil lead) which is under
tremendous pressure and heat. Sometimes, over a period of hundreds
of millions of years, the forming diamond absorbs trace elements
of minerals other than carbon. After it is mined, these "absorbed
minerals" may cause the diamond to show traces of another color
(usually blue) when exposed to strong ultraviolet light.
The effect of blue fluorescence on the appearance and price
of diamonds is one of the most controversial and confusing subjects
in the diamond business. A study of fluorescence conducted by
G.I.A. And published in the Winter 1997 edition of Gems and
Gemology (A Contribution to Understanding the Effects of
Blue Fluorescence on the Appearance of Diamonds) states:
"(when viewed by experienced observers) In the table up position
(as is commonly encountered in jewelry), diamonds described
as strongly or very strongly fluorescent were, on average, reported
as having a better color appearance than less fluorescent stones.
In this study, blue fluorescence was found to have even less
effect on transparency. These observations confirm G.I.A. GTL's
(Gem Trade Lab's) experience grading millions of diamonds over
the decades." Ironically, strongly and very strongly blue
fluorescent diamonds in better colors discount even though the
effect of fluorescence on the diamond's appearance is generally
positive.
Another
irony is that in the traditional color grading scale (pre-G.I.A.)
the top color grade for diamonds was "Jager" or "Blue-White."
This referred to perfect "D" color white diamonds with strong
blue fluorescence, which were regarded as a full color grade
above "D" color diamonds without fluorescence, because of their
beautiful blue-white brilliance. Many years ago you'd pay a
premium for a "blue-white." In today's market "blue-whites" discount approximately 10% from D colored stones without fluorescence.
Acceptable grades for Fluorescence:
None, Faint Blue, Medium Blue. In many cases, Strong or Very
Strong Blue are also acceptable but only if they've been evaluated
by an experienced gemologist whose opinion you trust to ensure
that the stone you are considering doesn't appear milky in certain
types of light. If you buy a diamond of I color or below, fluorescence
is normally regarded as a positive attribute, the stronger the
fluorescence, the better (because the diamonds appear to be
whiter than they actually are). Do not buy diamonds with yellow
fluorescence. Even very white diamonds appear yellow when they
have yellow fluorescence.
- Generally
the comments do not have a significant impact on the value of
the diamond. For the most part they note inclusions
or surface characteristics that are not noted in the plot of
the diamond (i.e., a small diagram of the diamond which maps
the inclusions and blemishes). Typical comments are: pinpoints
not shown; additional clouds not shown; surface graining is
not shown; etc.
However,
beware of comments pertaining to the crown angles, proportions
or cut of the diamond which could affect the play of light in
the stone. Any diamond containing this type of remark
should not be purchased. Crown angle comments are the most common,
for example: "Crown angle is less than 30 degrees". Do not buy
diamonds with comments pertaining to the crown angle.
I
generally recommend against buying diamonds when internal graining
is noted in the comments (typically referred to simply as "graining").
Unlike minor surface graining, significant internal graining
can affect the light transmission in the diamond and affect
it's brilliance. Also do not buy diamonds
noting laser treatment (i.e., some diamonds have drill holes
where inclusions have been eliminated with a laser - these diamonds
sell for substantial discounts). Dimonz company does
not sell diamonds with crown angle comments or laser treatments.
IMPORTANT
NOTE:
If
you wish to accurately evaluate the quality and value of a diamond
you are considering, you also need to know the crown angle and
the pavilion depth/angle of the stone (please refer to the discussion
under Cut). This information
is not contained on a GIA report:
CROWN
ANGLE (not recorded on the G.I.A. Report) -- is the angle formed where the girdle intersects with the
crown (i.e., the top part of the diamond). This angle forms
a sort of prism that changes the white light reflected from
inside the diamond through the crown into the multicolored "fire." If the crown angle is too shallow or too deep, the fire of the
diamond is diminished. The American
Gem Society (A.G.S.) maintains that an "ideal cut" diamond should
have a crown angle of 33.7-35.8 degrees. A Class I cut (G.I.A.'s
top rating) must have a crown angle from 34-35 degrees. Well
cut diamonds should generally have crown angles in the 32.5
to 36 degree range.
PAVILION ANGLE (not recorded on the G.I.A. Report)
-- This angle, which correlates directly to the pavilion depth,
is the most important measurement in determining the brilliance
of a diamond. If the pavilion angle is too shallow, the diamond
is less brilliant because of light leakage (Diagram C, above,
far right figure). If the pavilion angle is too steep, you also
get light leakage (Diagram, above, middle figure). In
conjunction with having a pavilion depth of 42.2%-43.8%, A.G.S.
Maintains that a diamond must have a pavilion angle of 40.15-41.20
degrees to be considered "ideal." Tolkowsky's "American
Ideal Cut" diamond has a pavilion angle of 40.75 degrees. G.I.A.,
to my knowledge, is silent on the issue of the proper pavilion
angle, instead relying solely on the pavilion depth of the stone
in assigning it's cut grades (see discussion, below). Well
cut diamonds should generally have pavilion angles of 40.15-41.60
degrees.
PAVILION
DEPTH PERCENTAGE (not recorded on the G.I.A. Report)
-- This measurement is just another way of expressing the pavilion
angle. It is the distance between the girdle and the culet (point
at the very bottom of the diamond) expressed a a percentage
of the girdle diameter of the diamond. For example, if the distance
between the girdle and the culet is 2.8 mm and the diamond has
a girdle diameter of 6.5 mm, the pavilion depth is 43% (2.8
mm divided by 6.5 mm = .43 or 43%). The pavilion depth, like
the pavilion angle, determines how much of the light which enters
the diamond is reflected back to the eye as brilliance. If the
pavilion depth is too shallow or too deep, the diamond is less
brilliant because less light is reflected back to the eye (see
the middle and far right figures in Diagram, above, i.e., either
the light leaks out of the back of the stone or at an angle
where it can't be seen). The A.G.S.
Maintains that the pavilion depth for an "ideal cut" diamond
should be between 42.2%-43.8%. G.I.A.'s Class I cut classification
requires a pavilion depth of 43% when rounded to the nearest
1/2% (i.e., 42.75%-43.25%). Well cut diamonds should generally
have a pavilion depth percentage of 44.2%-44.3%.
_______________________________________________________________________
HOW
TO READ A G.I.A. REPORT FOR A ROUND BRILLIANT DIAMOND (as noted
previously this information is also contained on A.G.S. reports
in a slightly different format)
1.
Shape and Cutting Style- Round Brilliant
2.
Measurements- the average of the first two measurements
(i.e., the max. min. width of the diamond) should be approximately
as follows:
| |
Ave. |
Approx.
Acceptable Range |
| .50 carat |
5.2 mm |
(5.1mm-5.3mm) |
| .75 carat |
5.9 mm |
(5.75mm - 6.0mm) |
| .85 carat |
6.2mm |
(6.05mm-6.3mm) |
| 1.00 carat |
6.5mm |
(6.4mm-6.6mm) |
| 1.25 carats |
7.0mm |
(6.85mm-7.1mm) |
| 1.50 carats |
7.4mm |
(7.25mm-7.5mm) |
| 1.75 carats |
7.8mm |
(7.6mm-7.95mm) |
| 2.00 carats |
8.2mm |
(8.0mm-8.35mm) |
| 2.25 carats |
8.6mm |
(8.45mm-8.75mm) |
| 2.50 carats |
9.0mm |
(8.75mm-9.2mm) |
3.
Weight- Weight in Carats (one carat=1/5 gram)
4.
Depth- 59%-63% is recommended
5.
Table*- 53%-62% is recommended (53%-60%= Ideal/Superior/61%-62%=
Good)
6.
Girdle Thickness- Do not buy a diamond that has as
one of it's measurements "extremely thin", "very thick" or "extremely
thick". Also do not buy a diamond that has a single measurement
of "thick". All other measurements are acceptable.
7.
Culet - none, very small, small, or medium are acceptable.
FINISH
8.
Polish* - Good, Very Good or Excellent are acceptable.
9.
Symmetry* - Good, Very Good or Excellent are acceptable.
10.
Clarity Grade- (Please see discussion under "Clarity" on this web-site. Generally, I recommend VS1, VS2 or SI1 clarity
as representing the best value).
11.
Color - (please see discussion under "Color" on this
web-site. Generally, I recommend G,H or I color as representing
the best value).
12.
Fluorescence - None, Faint Blue or Medium Blue are
acceptable. Strong or Very Strong Fluorescence may also be acceptable
depending on whether or not the diamond appears "milky" in ultraviolet
light (you should only buy a stone with Strong or Very Strong
Blue Fluorescence after consulting an experienced gemologist
that you trust). For stones of I color and below, fluorescence
is usually desirable because they appear whiter than their color
would indicate.
13.
Comments - Comments simply noting inclusions not
included on the plot are acceptable (e.g., pinpoints or clouds
not shown). Comments noting "surface graining" are acceptable
but those noting internal graining (referred to simply as "graining")
are generally unacceptable. Crown angle comments or others pertaining
to cut and/or comments noting a laser drill hole(s) are also
unacceptable.
NOTE:
AS PREVIOSLY STATED, G.I.A. REPORTS ARE INCOMPLETE WITHOUT A
SARIN BRILLIANT-EYE OR MEGASCOPE REPORT NOTING THE CROWN ANGLE
AND THE PAVILION ANGLE/DEPTH. WITHOUT THIS INFORMATION YOU CANNOT
ACCURATELY EVALUATE THE QUALITY OR VALUE OF THE DIAMOND YOU
ARE CONSIDERING. DESIRED PARAMETERS FOR THESE ELEMENTS ARE:
Crown
Angle-- The American Gem Society (A.G.S.) Maintains
that an "ideal cut" diamond should have a crown angle of 33.7-35.8
degrees. A Class I cut (G.I.A.'s top rating) must have a crown
angle from 34-35 degrees. Well cut diamonds should generally
have crown angles in the 32.5 to 36 degree range.
Pavilion
Angle-- In conjunction with having a pavilion depth
of 42.2%-43.8%, A.G.S. Maintains that a diamond must have
a pavilion angle of 40.15-41.20 degrees to be considered "ideal." Tolkowsky's "American Ideal Cut" diamond has a pavilion angle
of 40.75 degrees. G.I.A., To my knowledge, is silent on the
issue of the proper pavilion angle, instead relying solely on
the pavilion depth of the stone in assigning it's cut grades. Well cut diamonds should generally have pavilion angles of
40.15-41.60 degrees.
Pavilion Depth-- The A.G.S. Maintains that the
pavilion depth for an "ideal cut" diamond should be between
42.2%-43.8%. G.I.A.'s Class I cut classification requires a
pavilion depth of 43% when rounded to the nearest 1/2% (i.e.,
42.75%-43.25%). Well cut diamonds should generally have a pavilion
depth percentage of 44.2%-44.3%.
*Note: Diamonds with ratings of Very Good or Excellent for Polish and
Symmetry on the G.I.A. report (or A.G.S. "0" or "Ideal" on an
A.G.Ss. report) in combination with a table of 53%-60%, that
meet the Class I or Ideal cut criteria (including proper crown
angle and pavilion angle/depth measurements) generally sell
for a 10-15% premium over well-cut stones as defined herein
(please see the Cut section
of this website-- refer to the discussion of G.I.A. Class I
cut grade and A.G.S. "Ideal" or "000" cut grade).
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