Jewelry and Gems The Buying Guide: Diamond Grading Report

Today, few fine diamonds over one carat are sold without a diamond grading report, or certificate, as they are also called, from a respected laboratory. Reports issued by the GIA/Gem trade laboratory are most widely used in the United States and many countries around world.

A grading report does more than clarify the stone’s genuineness, it fully describes the stone and evaluate each of the critical factors affecting quality, beauty, and value. Grading reports can be very useful for a variety of reasons. The information they contain can provide verification of the “facts” as represented by the seller and enable one to make a safer decision when purchasing a diamond. Another, important function of reports is to verify the identity of a specific diamond at some future time, if, for example, it has been out of one’s possession for any reason. For insurance purposes, the information provided on the report will help ensure replacement of a lost or stolen diamond with one that is truly “compatible quality.”

Reports are not necessary for every diamond, and many beautiful diamonds used in jewelry are sold without them. But when considering the purchase of a very fine diamond weighting one carat or more, we strongly recommend that the diamond be accompanied by a report, even if it means having a diamond removed from its setting (no reputable lab will issue a report on a mounted diamond), and then reset. If you are considering a diamond that lacks a report, it is easy for your jeweler to obtain one. Or, now that GIA is issuing diamond grading reports to the public, you may submit a diamond at GIA yourself.

Do not rely on the report alone

The availability and widespread use of diamond grading reports can, when properly understood, enable even those without professional skills to make valid comparisons between several stones, and thus make more informed buying decisions. Reports can be an important tool to help you understand differences affecting price. But we must caution you not to let them interfere with what you like or really want. Remember, some diamonds are very beautiful even though they don’t adhere to establish standards. In the final analysis, use your own eyes and ask yourself how you like the stone.

A customer who was trying to decide between several diamonds. Her husband wanted to buy her the stone with the best report, but she preferred another stone which, according to what was on the reports, wasn’t as good. They decide against the best diamond and bought the one that made her happiest. The important thing is that they knew exactly what they were buying, and paid an appropriate price for that specific combination of quality factors. In other words, they made an informed choice. The reports gave them assurance as to the facts, and greater confidence that they knew what they were really comparing.

Improper use of reports can lead to costly mistakes

As important s diamond grading reports can be, they can also be misused and lead to erroneous conclusions and costly mistakes. The key to being able to rely on a diamond report, and having confidence in your decision, lies in knowing how to read it properly. For example, when trying to decide between two diamonds accompanied by diamond grading reports, buyers all too often make a decision by comparing just two factors evaluated on the reports, color and clarity, and think they have made a sound decision. This is rarely the case. No one can make a sound decision based on color and clarity alone. In fact, when significant price differences exists between two stones of the same color and clarity as the more expensive stone, and often it is not the better value. Having the same color and clarity is only part of the total picture. Differences in price indicates differences in quality, differences you may not see or understand. With round diamonds, the information you need is on the report, but you need to understand what all the information means before you can make valid comparisons.

A word of caution: Do not make a purchase relying solely on any report without making sure the report matches the diamond, and that the diamond is still in the same condition described. Always seek a professional gemologist, gemologist-appraiser, or gem-testing laboratory to confirm that the stone accompanying report is, in fact, the stone described there, and that the stone is still in the same condition indicated on the report. There are instances where a report has been accidentally sent with the wrong stone. And, in some cases, deliberate fraud is involved.

How to read a diamond grading report

Check the date issued. It is very important to check the date on the report. It’s always possible that the diamond has been damaged since the report was issued. This sometimes occurs with diamonds sold at auction. Since diamonds can become chipped or cracked with wear, one must always check them. For example, you might see a diamond accompanied by a report describing it as D – Flawless. If this stone were badly chipped after the report was issued, however, the clarity grade could easily drop to VVS, and in some cases, much lower. Needless to say, in such a case value would be dramatically reduced.

Who issued the report? Check the name of the laboratory issuing the report. Is the report from a laboratory that is known and respected? If not, the information on the report may not be reliable. Several well-respected laboratories issue reports on diamonds. The best known in the United States include the Gemological Institute of America Gem Trade Laboratory (GIA/GTL or GIA), and the American Gemological Laboratories (AGL). Respected European labs issuing reports include the Belgian Diamond High Council (HRD). Regardless of which report you are reading, all will provide similar information, including:

Identity of the stone. This verifies that the stone is a diamond. Some diamond reports don’t make a specific statement about identity because they are called diamond reports and are only issued for genuine diamonds. If the report is not called a “diamond grading report” then there must be a statement attesting that it is genuine diamond.

Weight. The exact carat weight must be given.

Dimensions. Any diamond, of any shape, should be measured and the dimensions recorded as a means of identification, especially for insurance/identification purposes. The dimensions given on a diamond report are very prices and provide information that is important for several reasons. First, the dimensions can help you determine that the diamond being examined is, in fact, the same diamond described in the report, since the likelihood of having two diamonds with exactly the same carat weight and millimeter dimensions is remote. Second, if the diamond has been damaged and re-cut since the report was issued, the millimeter dimensions may provide a clue that something has been altered, which might affect the carat weight as well. Any discrepancy between the dimension that you or your jeweler get by measuring the stone, and those provided on the report, should be a red flag to check the stone very carefully.

Finally, the dimensions on the report also tell you whether the stone is round or out of round. Out of round diamonds sell for less than those that are more perfectly round.

Fine diamonds are “well-rounded”.

The diamond’s roundness will affect value, so it is determined very carefully from measurements of the stone’s diameter, gauged at several points around around the circumference. For a round diamond, the report will usually give two diameters, measured in millimeters and noted to the hundredth: for example, 6.51 rather than 6.5; or 6.07 rather than 6.0. These indicate the highest and lowest diameter. Diamonds are very rarely perfectly round, which is why most diamond reports will show two measurements. recognizing the rarity of truly round diamonds, some deviation is permitted, and the stone will not be considered “out of round” unless it deviates by more than the established norm, approximately 0.10 millimeter in a one carat stone. In a one carat diamond, if the difference is 0.10 or less, then the stone is considered “round.” If the difference is greater, it is “out-of-round.”

To calculate an acceptable deviation on a particular stone, average the high and the low diameter dimension given and multiply that number by 0.0154. For example, if the dimensions given are 8.20x 8.31, the diameter average is 8.25 ( (8.20 + 8.31)/2). Multiply 8.25 by 0.0154 = 0.127. This is the acceptable deviation allowable for this stone (between 0.12 and 0.13). The actual deviation in this example would be 0.11 (8.31 – 8.20), well within the tolerance, so this diamond would be considered “round.” Some flexibility is permitted on diamonds over two carats.

Depending on degree of out-of-roundness (how much it deviates from being perfectly round), price can be affected. The greater the deviation, the lower the price should be.

Dimensions for fancy shapes

While dimension for fancy shapes diamonds are not as important as they are for round diamonds, there are length to width ratios that are considered “normal” and deviations may result in price reductions. The following reflect acceptable ranges:

Pear shape: 1.50:1 to 1.75:1

Marquise shape: 1.75:1 to 2.25:1

Emerald shape: 1.50:1 to 1.75:1

Oval shape: 1.50:1 to 1.75:1

To better understand what this means, let’s look at a marquise diamond as an example. If its report showed the length to be 15 millimeters and the width to be 10 millimeters the length to width ratio would be 15 to 10 or 1.5:1. This would be acceptable. If, however, the dimensions were 30 mm long by 10 mm wide, the ratio would be 30 to 10 or 3:1. This would be unacceptable; the ratio is too great, and the result is a stone that looks much too long for its width. Note: A long marquise is not necessarily bad, and some people prefer a longer shape, but it is important to understand that such stones should sell for less than those with normal lengths. Always keep in mind the length to width ratio of fancy cuts, and adjust the price for that are not in the acceptable range.

Evaluating proportioning from the report

As discussed earlier, good proportioning is as critical to diamond as it is to the man or woman who wears it! The proportioning, especially the depth percentage and table percentage, s what determines how brilliance and fire the stone will have.

The information provided on diamond reports pertaining to proportions is critically important for round, brilliant cut diamonds. Unfortunately, it is only of minimal use with fancy fancy shape diamonds. For fancies, you must learn to rely on your eye to tell whether or not the proportioning is acceptable: are there differences in brilliance across the stone? Or flatness? Or dark spots such as “bow-ties” resulting from poor proportioning.

Evaluating the proportioning of a diamond is as critical as evaluating the color and clarity grades. Diamonds that are cut close to “ideal” proportions, stones with “excellent” makes can easily cost more than the norm while diamonds with poor makes sell for less; very badly proportioned stones should be priced for much less. The information on a diamond report can help you evaluate the proportioning and know whether or not you should be paying more, or less, for a particular diamond.

Depth percentage and Table percentage key to beauty

To determine whether or not a round stone’s proportioning, so critical to its beauty, is good, look at the section of the report that describes depth percentage and table percentage. The depth percentage represents the depth of the stone, the distance from the table to the culet, as a percentage of the width of the stone. The table percentage represents the width of the table as a percentage of the width of the entire stone. These numbers indicate how well a round stone has been cut in terms of its proportioning, and must adhere to very precise standards. Your eye may be able to see differences in sparkle and brilliance, but you may not be able to discern the subtleties of proportioning. The percentages on the report should fall within a fairly specific range in order for the stone to be judged acceptable, excellent, or poor.

Some reports also provide information about the crown angle. The crown angle tells you the angle at which the crown portion has been cut. This angle will affect the depth and table percentage. Normally, if the crown angle is between 34 and 36 degrees, the table and depth will be excellent; between 32 and 34, good; between 30 and 32 degrees, fair; and less than 30 degrees, poor. If the exact crown angle is given, it is probably considered acceptable. If not, there is a statement indicating that crown angle exceeds 36 degrees, or is less than 30 degrees.

Depth percentage

A round diamond cut with a depth percentage between 58 and 64 percentage is normally a lovely, lively stone. You should note, however, that girdle thickness will affect depth percentage. A high depth percentage could result from a thick or very thick girdle, so when checking depth percentage on the diamond report, check the girdle information as well.

Stones with a depth percentage over 64% or under 57% will normally be too deep or too shallow to exhibit maximum beauty and should sell for less. If the depth percentage is too high, the stone will look smaller than its weight indicates. If the depth percentage is exceptionally high, brilliance can be significantly affected. Diamonds that are so shallow, that is, stones with such low depth percentages, that they have no brilliance and liveliness at all. When dirty, such stones look no better than a piece of glass.

We avoid diamonds with depth percentages over 64% or under 57%. If you are attracted to such diamonds remember that they should sell for much less per carat.

Table Percentage

Round diamonds cut with tables ranging from 53% – 64% usually result in beautiful, lively stones. Diamonds with smaller tables usually exhibit more fire than those with larger tables, but stones with larger tables may have more brilliance. As you see, table width affects the diamond’s personality, but deciding which personality is more desirable is a matter of personal taste.


Under finish on the diamond report, you will find an evaluation of the diamond’s polish and symmetry. Polish serves as an indicator of the care taken by the cutter. The quality of the stone’s polish is a factor that can not be ignored in evaluating the overall quality of a diamond, as well as its cost and value. Polish can be described on the report as excellent, very good, good, fair, or poor. The price per carat should be less on diamonds with “fair” or “poor” polish. Cost per carat is usually more for diamonds that have “very good” or “excellent” Polish.

Symmetry describes several factors:

How the facet edges align with one another;

whether or not the facets from one side of the diamond match corresponding facets on the opposite side;

whether or not facets in the top portion of the diamond are properly aligned with corresponding ones in the bottom portion.

When the symmetry is described as “fair”, or worse, something is out of line.

When evaluating symmetry, the most important area to check is the alignment of the crown (top) to the pavilion (bottom). If it is not good, it will make a visual difference in the beauty of the stone, and correspondingly in its price. To check for proper alignment here, simply look at the diamond from the side to see whether or not the facets just above the girdle align with the facets just beneath the girdle.

When the top and bottom facets do not line up, it indicates sloppy cutting and, more important, the overall beauty of the diamond’s is diminished. This will reduce the price more than other symmetry faults.

How does the girdle affect value?

The girdle is another important item described on diamond grading reports. The report will indicate whether or not the girdle is polished, or faceted, and how thick it is. Girdle thickness ie very important for two reasons:

It affects value, and

It affects the diamond durability.

Girdle thickness ranges from extremely thin to extremely thick. Diamonds with girdles that are excessively thin or thick normally sell for less than other diamonds. An extremely thin girdle increases the risk of chipping. remember that despite their legendary hardness, diamonds are brittle, so very thin edge poses a greater risk.

If a diamond has an extremely thick girdle, its cost should also be reduced somewhat because the stone will look smaller than another diamond of the same weight with a more normal girdle thickness. This is because extra weight is being consumed by the thickness of the girdle itself.

There are some cases in which a very thick girdle is acceptable. Shapes that have one or more points, such as the pear shape, heart, or marquise, can have thick to very thick girdles in area of the points and still be in the acceptable range. Here the extra thickness in the girdle helps protect the points themselves from chipping.

Generally, a diamond with an extremely thin girdle should sell for less than one with an extremely thick girdle because of the diamond’s increased vulnerability to chipping. However, if the girdle is much too thick (as in older diamonds), the price can also be significantly less because the stone cam look significantly smaller than other diamonds of comparable weight.

The Culet

The culet looks like a point at the bottom of the diamond, but it is normally another facet, a tiny, flat polish surface. This facet should be small or very small. A small or very small culet won’t be noticeable from the top. Some diamonds, today, are pointed. This means that there really is no culet, that the stone has been cut straight down to a point instead. The larger the culet, the more visible it will be form the top. The more visible, the lower the cost of the diamond. Diamond described as having large or “open” culet as in old European or old-mine cut diamonds are less desirable, because the appearance of the culet causes a reduction in sparkle or brilliance at the very center of the stone. These stones normally need to be re-cut, and their price should take the need for re-cutting. for the same reasons, a chipped or broken culet will seriously detract from the stone’s beauty and significantly reduce the cost.

Color and Clarity

The color and clarity grades on a diamond report are the items most people are familiar with. They are important factors in terms of determining the value of a diamond, but as the preceding discussion has shown, they do not tell the whole story about the diamond.

A word about fluorescence

Fluorescence, if present, will also be indicated on a diamond grading report. It will be graded weak, moderate, strong, or very strong. Some reports indicate the color of the fluorescence as blue, yellow, white, and so on. If fluorescence is moderate to very strong and the color is not indicated, you should ask the jeweler to tell you what color the stone fluoresces. A stone with strong yellow fluorescence should sell for less since it will appear more yellow than it really is when worn in daylight or fluorescent lighting. The presence of blue fluorescence will not detract, and in some cases may be considered a bonus since it may make the stone appear more white than it really is in daylight or fluorescent lighting. However, if the report show a very strong blue fluorescence, there may be an oily or milky appearance to the diamond. If the stone appears milky or oily to you as you look at it, especially in daylight or fluorescent light, it should sell for less.

Pay attention to the full clarity picture provided

The placement, number, type, and color of internal and external flaws will be indicated on a diamond grading report, may include a plotting, d diagram showing all the details. Be sure you carefully note all the details in addition to the cumulative grade. Remember, the placement of imperfections can affect value.

A reliable diamond grading report cannot be issued on a fracture-filled diamond, so GIA and most other labs will not issue a report on diamonds that have been clarity enhanced by this method. The diamond will be returned with a notation that it is filled and cannot be graded. Reports are issued on diamonds that have been clarity enhanced by laser. Remember, however, that no matter what the clarity grade, a lasered diamond should not cost less than another with the same grade.

A final word about diamond reports

Diamond grading reports provide a very useful tool to aid in comparing diamonds and evaluating quality and value. But the key to their usefulness is proper understanding of how to read them, and how to look at the stone. Those who take the time to learn and understand what they are reading and, therefore, what they are really buying, will have a major advantage over those who do not.