Cooper Jewelers - Warren NJ - Education

Diamond Cut

Diamond cutting can be traced back to the mid 1300s when European and Indian gem cutters first began to cut and shape rough diamonds. Since that time, there have been many cutting methods identified and published.

Of the 4Cs, Gemologists believe that cut has the greatest influence on a diamond's beauty. Diamonds have a unique ability to effectively manipulate light. This unique quality can only be realized with an extremely high level of accuracy during the cutting and polishing process. Where nature dictates the uniqueness of color and clarity, humans affect the cut. While cutting diamond rough, cutters must not only consider the proportions of a diamond, but also the craftsmanship of overall symmetry and polish as well.

A diamond's brilliance comes from light entering the crown and reflecting from one facet to another and returning back out the crown. A diamond that is cut too shallow or too deep will not reflect light properly and the diamond will not be as brilliant as a diamond with an excellent cut.

How a Diamond Handles Light

Cooper Jewelers - Warren NJ - DIAMOND perfect cut, deep cut, shallow cut

GIA and Diamond Cut

In early 2005, after more than 15 years of research and testing, the Gemological Institute of American (GIA) introduced a diamond cut grading system for round brilliant shaped diamonds in the D-Z color range. Starting in January 2006, the new cut grades ranging between excellent and poor were added to all GIA diamond certificates (aka plot). Each cut grade represents a range of proportions and diamond appearances each of which have an impact to the characteristics of that grade.

To achieve the cut grading system, GIA calculated the cut results for 38.5 million proportion sets based on the evaluation of seven components. The first three, which were related to appearance were brightness, fire, and scintillation while the second four, weight ratio, durability, polish, and symmetry were related to the overall craftsmanship.

To understand all aspects of the diamond anatomy, click on the GIA link here.

GIA certificates issued before 2006

GIA certificates issued before 2006 do not display cut grades. Until that time, the term Ideal cut was considered the best cut for a round brilliant cut diamond. The GIA study concluded that many combinations or parameters made for an equally efficient cut and in some cases certain ideal parameters were inferior to the GIA excellent cuts.


Understanding Diamond Proportions

Round Brilliant Cut Diamonds

A diamond's proportions and their relationship to each other primarily influence the GIA cut grade assigned. Grading the cut of a polished diamond begins with accessing brightness, fire, and scintillation. Every proportion of a round brilliant contributes to its interaction with light. If one proportion is "off" even slightly, a diamond could receive a lower cut grade. To understand cut grade it is important to know how proportions are evaluated and their relationships to one another.

Cooper Jewelers - Warren NJ - DIAMOND perfect cut, deep cut, shallow cut

Grading Proportions - Crown, Table, and Girdle

(Diamonds are always measured in millimeters)

Average Girdle Diameter

Determining average girdle diameter is the first step in evaluating proportions and is the starting point for comparing the diamond's other proportions. To determine average girdle diameter, measurements are taken from one edge of the girdle to the other across the diamond in several locations. Since no diamond is completely round it is necessary to take several measurements. After determining the measurements, add the smallest and largest numbers, divide by two, and round to the nearest hundredth (0.01) of a millimeter.

Minimum Girdle Diameter = 3.54mm
Maximum Girdle Diameter = 3.57mm

3.54mm + 3.57mm = 7.11
7.11 divided by 2 = 3.555
3.555 rounded to the nearest hundredth mm = 3.56mm
(Source: GIA)

Total Depth Percentage

Total depth percentage determines why a diamond is underweight or overweight in relation to its diameter. After obtaining the diamond's average girdle diameter the total depth percentage can be calculated. To determine total depth percentage the diamond is measured from table to culet. After determining the measurement, divide the figure by the diamond's average girdle diameter, multiply by 100, and round to the nearest tenth of a percent.

Average Girdle Diameter = 6.52mm
Table to Culet Measurement = 4.02mm

4.02mm divided by 6.52mm = 0.61656
0.61656 x 100 = 61.656
61.656 rounded to the nearest tenth = 61.7%
(Source: GIA)

A well-proportioned diamond will have a total depth percentage of approximately 60.0%. Diamonds with a total depth percentage below 55.0% are usually under weight. Diamonds with a total depth percentage of 65.0% or greater are usually over weight and can vary greatly in terms of reasons.

Note - although a diamond may have a total depth percentage of 60.0%, which indicates a well-proportioned diamond, it is important to note that the diamond may still have negatives such as a shallow crown, deep pavilion, or thick girdle. Also, total depth percentage ranges overlap between grades.

Table Facet and Table Percentage

The largest cut facet on a diamond is the table. The table facet along with the other facets allows light to enter and exit a diamond. The size of the table facet is an important factor when determining proportions. Table size is stated as table percentage, which is a percentage of the diamond's average girdle diameter.

Table size preference differs in markets throughout the world, but despite these differences a very small table (below 50%) or very large table (70% and greater) will have a negative impact on the diamond's face up (crown up-pavilion down) appearance, which will ultimately have a negative impact on the cut grade. Most diamond cutters today will cut diamonds with a table percentage of between 55%-65%.

Star Facet and Star Facet Length Percentage

Star facets extend from the edge of the table toward the girdle. They are very important to consider when evaluating a diamond's proportions in relation to the rest of the crown as they affect both brightness and fire.

Star facet length percentage is the length of the star facets expressed as a percentage of the total distance between the girdle and the edge of the table facet. Star facet length percentages usually range from 50%-55%. Beyond that range, very long star facets average 65%-70% and very small start facets range below 35%, which indicates the diamond is not symmetrical and will receive a lower cut grade.

The Crown - Crown Angle and Crown Height Percentage

Crown angle and crown height percentage are the two crown proportions that affect the diamond's appearance. Crown angle is the angle formed by the bezel facets and the girdle plane while the crown angle height percentage is the distance from the girdle plane to the table expressed as a percentage of the average girdle diameter. Crown angles between 25 degrees and 35 degrees produce bright diamonds with much fire. Crown height percentage is determined by the diamond's table percentage and crown angle.

The Girdle

The Girdle of a diamond basically prevents damage and provides an "edge" for setting the stone. A girdle can be bruted, polished, or faceted. Girdles should be thick enough to prevent chipping, but not too thick where it adds extra weight to the diamond. Thicker unpolished girdles can created a grayish reflection in the stone which can be unattractive and a girdle that is too thin can create durability problems as well as issues when setting.

Girdle thickness is usually reported in a range for example; thin-to-thick or thin-to-medium. Also, you may have heard of the terms "painting" and "digging out" both of which are weight saving techniques. Painting is a method to adjust the angles of facets to allow portions of the girdle to be thicker and digging out is a method leaving more girdle thickness to allow for clarity characteristics to be removed at a later date.


Diamond "Color" History

The term "color" in white diamonds actually refers to the lack of color present. Diamonds composed of pure carbon are colorless and extremely rare and costly. Most diamonds contain nitrogen, boron, or hydrogen all of which impact color. A majority of white diamonds sold on the market today contain traces of nitrogen, which causes slight shades of yellow or brown. Small, subtle differences in color can make a substantial difference in a diamond's value.

Diamond Color Grading System

In a effort to eliminate confusion related to diamond color, the Gemological Institute of America (GIA) implemented the color grading system in the 1950s which is still being used industry wide today. Diamonds are graded on a scale beginning with the letter D (colorless) and ending with Z (light yellow or brown). When creating the new color scale GIA chose to start with the letter D as a means of starting over. Prior to GIA implementing the D-Z scale, other systems all of which were inconsistent and inaccurate, already used the letters A-C, numbers 0-3, and the Roman numerals I-III.

It is important to note that diamonds at the end of the color range with a noticeable yellow tinge are not considered fancy yellow color diamonds.

Cooper Jewelers - Warren NJ - DIAMOND color

Recommendations for choosing a Diamond Color

Many factors should be considered when deciding on color. You should always look for a diamond that is as white as possible within your budget.

  • When a diamond is well cut the diamond's refraction and dispersion will often disguise certain degrees of color and may make a darker diamond appear whiter when viewed with the unaided eye (without magnification).

  • The size of the diamond will also affect the appearance of its color. Color can easily be seen in larger diamonds as opposed to smaller diamonds. Often times it is difficult to see color in diamonds under a 1/2 carat with the unaided eye. Gemologists use magnification to determine the difference in color of two otherwise similar diamonds that may be two or three color grades apart. In larger size diamonds it does become easier; however while there may be a very fine disparity between one color grade to the next, the cost can be significant. If a larger diamond is a priority, you should consider sacrificing slightly on color.

  • The shape of the diamond will affect its color as well particularly in step cut diamonds such as the Asscher and Emerald that have large open facets and do not display the brilliance of other cuts. The center of these diamonds display a "window effect" where some of the color in the diamond can appear washed out and make it appear whiter then its actual color grade. The shallower the cut of the diamond the more apparent this will be.


One additional factor in grading the color of a diamond is fluorescence. Fluorescence is the emission of visible light by a diamond when it is exposed to ultraviolet radiation. Approximately 10% of all diamonds on the market today exhibit fluorescence. Years ago, these diamonds were referred to as blue white diamonds. Today, fluorescence on a diamond certificate is divided into five different categories: none, faint, medium, strong, and very strong. Most diamonds that do fluoresce do so in a blue color; however, yellow and other colors are possible.

Fluorescence can be both a negative and a positive. In the strong and very strong range a diamond may appear oily or cloudy which will greatly reduce the attractiveness and value of the diamond. However, diamonds of I color and lower can actually look whiter with some fluorescence which; enhances their value.


Diamond Clarity

In addition to developing and implementing the color scale, GIA also developed a scale for diamond clarity. This was done to create consistency within the industry. In the past, jewelers used terms that were easily misinterpreted such as "loupe clean." The terms used today are done so worldwide.

The GIA clarity scale consists of 11 grades ranging from flawless to included. When determining a diamond's clarity grade, GIA considers the size, nature, color, position, and quantity of clarity characteristics under 10x magnification. The lesser the number of inclusions or blemishes a diamond has the higher clarity grade it will receive. A diamond with no inclusions using 10x magnification will be considered flawless and is very rare and more costly.


An inclusion is a clarity characteristic either totally enclosed in a polished diamond, reaching or extending into it from the surface, or one that is caused by treatments or the cutting process. Note - Inclusions caused by treatments or the cutting process may not always be documented on the diamond certificate.

Click here to view diamond inclusion information.


A blemish or external clarity characteristic is on the surface of the diamond only and can be caused by wear, the cutting process, or may be a result of the diamond's crystal structure. Blemishes play a lesser role than inclusions do when determining the clarity grade, but may affect the polish grade.

Click here to view diamond blemish information.

Grading Scale and Diamond Clarity Recommendations

Cooper Jewelers - Warren NJ - DIAMOND clarity

 FL  Flawless - no blemishes or inclusions under 10x magnification
 IF  Internally flawless - no inclusions and only very minor surface blemishes
 VVS1  Very, very slightly included - very, very small microscopic inclusions extremely difficult to see under 10x magnification.
 VVS2  Very, very slightly included - very, very small microscopic inclusions extremely difficult to see under 10x magnification.
 VS1  Very slightly included - very small microscopic inclusions difficult to see under 10x magnification.
 VS2  Very slightly included - very small inclusions somewhat easy to see under 10x magnification. These diamonds represent a good value to someone looking for a high quality diamond, as the very small imperfections do not affect the beauty of the diamond.
 SI1  Slightly included - small inclusions that are easy to see under 10x magnification. This clarity is an excellent choice for someone looking to stay within a budget, but wants a diamond that will look as good as a higher clarity diamond except when viewed under magnification.
 SI2  Slightly included - small inclusions that are visible under 10x magnification. Depending on their location within the diamond, they may sometimes be visible to the unaided eye without magnification. SI2 diamonds are attractively priced and typically no visible difference can be seen without magnification.
 I1*  Included - imperfect with inclusions that are obvious to the unaided eye and may affect the diamond's durability.
 I2*  Included - imperfect with inclusions that are obvious to the unaided eye and may affect the diamond's durability.
 I3*  Included - imperfect with inclusions that are extremely obvious to the unaided eye and pose a definite threat to the diamond's durability.