| HOW TO READ G.I.A. REPORTS |   |
In my opinion, Gemological Institute of America (G.I.A.) diamond grading reports are the most accurate and reliable available. I rarely use other reports. All of the information you need to evaluate the quality of a round brilliant cut diamond is contained in a G.I.A. Report. Please read the segment of this website on Cut under the "Four C's" section of this website, before reading this section.
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 without professional assistance (i.e., cushions, asscher cut, emerald cut, pear shapes, ovals, marquises, princesses, radiants, heart-shaped, etc.).
HOW TO READ A DIAMOND GRADING REPORTS FOR ROUND BRILLIANT CUT DIAMONDS FROM THE GEMOLOGICAL INSTITUTE OF AMERICA
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
SHAPE AND CUTTING STYLE - As stated, this field describes the shape and cutting style of the diamond (e.g., "round brilliant").
MEASUREMENTS - 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 | 6.1mm | (6.0mm-6.2mm) |
| 1.00 carat | 6.5mm | (6.4mm-6.6mm) |
| 1.25 carats | 7.0mm | (6.85mm-7.1mm) |
| 1.50 carats | 7.4mm | (7.30m-7.55mm) |
| 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 | (8.80mm-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).
CARAT WEIGHT- Weight of the diamond in carats. A carat is equal to 1/5 of a gram.
COLOR - Please see discussion under "COLOR" on this web-site. Generally, I recommend G, H or I color as representing the best value.
CLARITY GRADE- Please see discussion under "Clarity" on this web-site. Generally, I recommend VS1, VS2 or SI1 clarity as representing the best value.
CUT GRADE - There are five G.I.A. overall cut grades: Excellent, Very Good, Good, Fair and Poor. In my opinion, any round diamond you consider buying should have a cut grade of Excellent . The Excellent cut grade representsabout 20% of all round diamonds. If you want an extremely optically efficient stone, in addition to the Excellent cut grade, the author would also look for the following (represented in the diamond diagram in the lower right hand chart of the G.I.A. reort): a table percentage of 53%-60%; a crown angle from 33.7- 35.5 degrees; and a pavilion angle of 40.4-41.2 degrees. These measurements are more stringent than those currenly used by G.I.A. in assigning their Excellent cut grade. They are also more stringent than those currently used by the American Gem Society in assigning their Ideal cut grade. The table percentage, the crown angle, and the pavilion angle are discussed in detail, below.
FINISH (I.E., POLISH AND SYMMETRY)-- Each field under finish is discussed below
DEPTH - 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 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.
Diagrams of play of light
DIAGRAM A
TABLE -
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. are acceptable (e.g. a crown angle of 33.7 to 35.5 degrees; a pavilion angle of 40.2-41.2 degrees; an Excellent overall cut grade; 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. Dimonz generally does not sell round diamonds with a table larger than 60%.
61%-62% - Marginal- If all the other measurements of the diamond are within acceptable parameters, round diamonds with tables in this range demonstrate a reasonable amount of brilliance and fire.
63% and above - Poor - Round diamonds with tables larger than 63% generally display too little fire.
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.
CULET- 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
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).
CROWN ANGLE -- 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 -- 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 -- 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%).
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SUMMARY
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).
