Diamond Certification and Grading: Diamond Grading Reports
A Diamond Grading Report is a report or certificate issued by an independent Gemological Laboratory, which states that a diamond (usually over 1 carat) has been examined using various gemological instruments, compared to similar master diamonds, and has been found to contain the characteristics listed in the grading report. The grading report is usually referred to as a certification, although they are not the same; only Hoge Raad Voor Diamant (Diamond High Council, Belgium), HRD, calls their reports certificates. A Diamond Dossier is used by The Gemological Institute of America, GIA, for diamonds less than 1 carat and is also referred to as a certification. The American Gem Society Laboratories or AGS refers to their reports as Diamond Quality Documents. A diamond’s value is primarily derived from the characteristics on the diamond’s report and these aspects will vary from laboratory to laboratory. Always look at the date on a report. Older reports may indicate that a diamond is used and may have been set, removed, reset, and come in contact with other diamonds, which can contribute to the deterioration of a diamond’s polish. Older reports also contain less information. We have come across many diamonds that have been chipped and re-cut, but are still accompanied by the original report. Most diamonds come with reports because without one, a consumer would have to rely solely on a jeweler’s word. The Gemological Institute of America, GIA, is the largest and most well known laboratory in the world and as such, a diamond accompanied by a GIA report will cost more, although the diamond may not be worth more. GIA established the standards and methods for a diamond’s evaluation, which all labs now follow. They still own the highest quality and most advanced instruments in the world. See our grading labs section for more. If you find a diamond that isn’t accompanied by a grading report, ask your seller to send it to a reputable lab to have it evaluated. They will usually be more than willing as long as you cover the cost. If the seller has some hesitations it may indicate that the diamond is nowhere near the stated specifications or it may have come through a less than reputable source. If you decide not to purchase the diamond, it will now be worth more to the seller with a certificate. Bear in mind that an “uncertified” diamond can be more valuable than one that comes with an illegitimate grading report, especially if you don’t know the difference between labs. Many of these diamonds were once accompanied with reports from legitimate labs but have since been replaced by reports from much more lenient labs. Below are examples of an EGLUSA Diamond Certificate, a GIA Diamond Grading Report, and an AGS Diamond Quality Document.

Going Beyond the 4 Diamond C's: what else is on diamond certifications...

Measurements

The measurements of a diamond are primarily used for identification purposes, since no two diamonds are exactly alike in weight, quality and measurements together.

However, there is one way to use the measurements of a diamond to help you judge the potential beauty and brilliance, but it needs explaining.

For more info on this please see our brilliant cut measurement section.

For fancy shapes like the princess, marquise or pear shapes, the overall length-to-width ratio is important because certain proportions are more appealing to the eye.

To compute the length-to-width ratio of a fancy-shaped diamond ("fancy shape" means anything other than round brilliant), take the length of the diamond and divide by the width. Example: A marquise that measures 8 mm  x 5 mm has a 1.6:1 ratio. Most people just write or say it as a 1.6 ratio.

Depth Percentage

Depth percentage refers to the ratio of the average depth of the diamond (from table to culet) as compared with the average diameter (which is used as the 100% figure for all diamond dimensions). For instance, if the total diameter of a round diamond is 6.00 mm and the total depth is 4.00 mm, then the depth percentage is 66.7%.

The ranges that are commonly considered best for each shape vary slightly, but we will just give the basic dimensions for rounds here. Dimensions for other shapes are beyond the scope of this brief tutorial and should be discussed with a jeweler you trust.

Ideal = 58 - 60%
Excellent = 60.1 - 62%
Good = 62 - 64%
Fair = 64 - 66%
Poor = over 66% or under 57%

Table Percentage

Table percentage is the ratio of the table width as compared to the total width of the diamond. For rounds, the largest diameter is used. For fancies, the smallest diameter (width as opposed to length) is used. For example, if the table measures 5.7 mm, and the total diameter of the diamond measures 10.0 mm, then the table percentage is 57%.

It is important to note that the trend toward smaller table percentages is a very recent phenomenon. Until the last 5 years or so, most people preferred a slightly larger table because it added width to the stone without sacrificing brilliance. Today, the trend toward "ideal" is creating a great demand for diamonds that are cut to so-called "ideal" proportions set forth by AGS first and now other organizations, increasing the price for Ideal Cut diamonds. As evidence, see our Q&A section on Blue Nile.

Actually, perfect proportions are not possible because the proportions that would be perfect vary for each individual diamond depending on the physical nature of the diamond material and where it is mined. Each diamond has its own unique properties. Stones from four different mines can be cut to the exact same proportions but look very different.

Diamond Proportions

The proportions of a diamond can make an enormous difference in the appearance of a diamond. On a GIA certificate, Diamond "Proportion" is a general heading that includes the Depth and Table percentages and various other details as described below.

Some other labs (like AGS) include more information about the cut. You will notice (refer to your printed samples) that the AGS report includes every important detail about the cut, which is the most important factor in the beauty of a diamond. This is the only current weakness of the GIA report and the unique strength of the AGS reports.

Diamond Proportions, polish, and symmetry are actually more important than color, clarity or weight in determining beauty. Keep in mind that beauty is in the eye of the beholder, and everyone does not have exactly the same preferences in diamond proportions just as with color or shape. Some people prefer a table that is a little wider (within reason) to make the diamond appear larger and brighter, while others like a smaller table because it may add a little more fire. Look at several options and figure out your loved ones personal preferences.

Cutlet

The culet on a diamond is the tiny flat facet that is sometimes (but not usually) created by polishing off the tip at the bottom where all the facets of the pavilion otherwise come to a point. This used to be done to protect the fragile tip of the diamond from chipping off by accident, which would require an expensive recut. This is not such a common practice these days.

Diamonds without culets have become the norm. We recommend stones noted as having none, small, or very small culets. It is thought to make a diamond slightly more brilliant, but it is not crucial as long as it is small or nil. Do not make this a big issue. This is where you can leave a little leeway as you shop around for your diamond. If everything else about a diamond is fine, this should not be enough to make a difference.

Finish

The finish of a diamond includes two separate factors: the polish and symmetry. The finish is a very good indicator of the care taken by the craftsman to make the diamond as beautiful as possible. This is one of the most contributing factors to the cut grade and cut quality of a diamond. We do not recommend engagement diamonds with Poor or Fair grading for polish or finish.

Polish

Diamonds are capable of taking a polish better than any other material on earth. This means that a diamond can be polished to have the most perfect, reflective surfaces. This is one reason why diamonds are known for their incredible brilliance. Polish may deteriorate in used or pre-owned diamonds and should be something to watch out for.

Grades for polish range from Ideal (which is very rare) to Excellent, Very Good, Good, Poor, and Fair. We consider Good polish to be the minimum for engagement rings. Polish is not as important as symmetry, though, so don't get too hung up on this detail. Most other parameters will affect the visual properties of the stone a lot more than polish, as long as it is rated at least Good.

Symmetry

We consider symmetry fairly important in the making of a beautiful diamond. Without symmetry, the facets of the upper part of the diamond (called the crown) and the lower part (called the pavilion) may not match up. Poor facet alignment will allow light to leak out before it reflects back to your eye, resulting in less brilliance and fire. In other diamonds, such as a heart shape, poor symmetry may cause one side to be broader than the other and give it a lop-sided appearance, or it could result in uneven facet arrangements that allow light to leak and decrease the overall brilliance of the stone.

Grades for symmetry range from Ideal (which is very rare) to Excellent, Very Good, Good, Poor, and Fair. We consider Good symmetry to be the minimum for an engagement ring.

Fluorescence

Some diamonds have a natural blue fluorescence under ultraviolet light. This is natural.

How does fluorescence effect the beauty or value of a diamond? The answer to that question depends on the color grade you are buying.

For a diamond with very high color grade (such as a D to F), extremely strong blue fluorescence is thought to interfere with the flow of light and make the diamond appear a little oily or murky. But this really only applies to stones with D, E, or F color grades, and only if the fluorescence is extreme, which is VERY rare and not commonly seen in the marketplace at all. The effect of fluorescence is quite over-rated, so you can often ignore it except for extreme cases.

In grades just below the ones mentioned above (G, H and I color grades), moderate to even strong blue fluorescence can actually improve the color appearance to the naked eye in sunlight, so it might actually be a good choice. It helps counteract some of the yellow and make it appear more white or colorless. Such diamonds can be a great value because people don't know when fluorescence is beneficial.

Within the jewelry trade, fluorescence is often considered to slightly diminish the value of a diamond. This is because of supply and demand, as diamonds without any fluorescence at all are a little rarer than those with fluorescence.

Gridle

The girdle of a diamond is the middle, or widest part, of the diamond, and is measured from "extremely thin" to "extremely thick".

Thin girdles are prone to chipping. Extremely thick girdles hide a lot of weight, so you end up paying for a heavier diamond that doesn't look that big. The ideal is an even, medium girdle; this applies to all shapes except for the tip of the pear shape, in which a thick girdle is preferred to make it less prone to damage. When examining the girdle, make sure that it is well-faceted and polished, since some diamond cutters will omit this step to save time and to avoid removing weight from the stone.

Plot of Inclusions

The main thing most consumers want to know about clarity is very simple: "Does the diamond have any flaws I can see with my naked eye?"

Usually the answer to this question can be determined by looking at the map, or plot of inclusions, of the diamond.

A diamond with a clarity grade of VS2, VS1 or better, will not have any inclusions visible with the naked eye. However, it becomes a little harder to make that statement about an SI1 clarity grade, because placement of the inclusion will make a difference.

If the main inclusion in an SI1 is directly under the table facet (the large facet at the top center), you might be able to see it with the naked eye if you have very good vision and know where to look. So if you are looking at an SI2, just be sure the main inclusion is not under the table if you want to be totally safe.

If the clarity grade is VS2 or less, we recommend examining the plot to see the placement of the major inclusions.

Additional gemological terms such as feather, pinpoint, cloud, or natural are too complicated for this brief tutorial. Ask a gemologist you trust if you are curious. However, if you see any of these comments in the plot, do not be concerned. They are just technical terms for the types of inclusions in the diamond. These features are too small and insignificant to draw on the plot, so they just make a written comment for the sake of being thorough. It also helps identify the diamond, as part of the list of unique properties. The overall clarity grade is the most important factor, since that tells you the size of the inclusions (and therefore the visibility of the inclusions).

Date of the Certificate

The date on the certificate is more important than you might think. If you are considering buying a diamond with a certificate dated over 2 years ago, a lot could have happened to that diamond since it was examined by the laboratory. For instance, many diamonds are sold, set in a ring, worn by someone, and then sold to a jeweler a year later. While it was worn, the diamond could have been nicked or chipped, decreasing the clarity grade and diminishing the value of the stone somewhat.

Therefore, we like to suggest a rule. If a certificate is over 2 years old, you should at least ask the jeweler to explain where it has been during that time. We admit it is possible the diamond could have been in the vault for that long, but usually diamonds sell faster than that at a jeweler that specializes in selling engagement rings. Just ask and see what they say!

If the certificate is more than 2 years old, you might just want to avoid it, or suggest that the store send it out to have it graded by a lab again to make sure the information is still valid.