Jan 29

Jewelry and Gems The Buying Guide – Diamond Grading Report (2)

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.


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