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CUT DIAMONDS GUIDE

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CUT DIAMONDS GUIDE

CUT-- Please also read the section of this web-site titled "GIA CERTS"

Dimonz prices are the lowest anywhere for finely cut diamonds with reports from the Gemological Institute of America (G.I.A.) and the American Gem Society (A.G.S.).

The A.G.S. Report has all of the relevant information pertaining to the quality of the diamond, including it's cut, which is rated on a scale from 0 (ideal) to 10 (terrible). However, it must be noted that a G.I.A. report does not contain all of the information necessary to evaluate cut and does not assign a cut grade. All G.I.A. certified diamonds sold by Dimonz are accompanied by a Sarin Brilliant-Eye Report , which provides all of the specifics measurements you need to evaluate the cut of a diamond and assigns it a cut grade.
 

Due to the large number of requests I have recently received for "ideal cut" diamonds from people researching on the internet, the following report provides a detailed discussion of Cut, including a thorough discussion of what is meant by the term "ideal cut." I apologize for the technical nature of the discussion which follows. Unfortunately, there is no way of treating this subject adequately without going into some rather arcane technical detail. It is also necessary to point out that there is a great deal of controversy and disagreement among various gemological institutions with regard to the definition of what constitutes an "ideal cut" diamond and whether this term is even appropriate.

Of the 4 C's (cut, color, clarity and carat weight), most experienced gemologists believe that cut is the most important. Why? Because, in most cases, the cut of the diamond has more impact on it's brilliance and beauty than the other "C's". From the standpoint of aesthetics, a beautifully cut diamond of J color and SI2 clarity will put a flawless, D color diamond with an inferior cut to shame (although the D Flawless diamond may cost 3-5 times the price of the J SI2!!!).

The price of diamonds with the same carat weight, color and clarity may vary in price from 30% to 50% depending on the cut of the stone. The average diamond sold in the U.S. is poorly cut and lacks the brilliance and fire that you see in better cut diamonds. When pricing diamonds, be sure to compare the prices of well-cut diamonds only.

There are many "terms of art" pertaining to cut which shall be discussed in this section. You need to be familiar with all of them if you are to understand what constitutes a better cut more brilliant diamond. These include: the girdle diameter, table percentage, girdle thickness, depth percentage, polish and symmetry grades and culet size. All of this information is described on the G.I.A. Report.

However, you also need to know the diamond's crown angle and pavilion depth/angle before you can evaluate how well it is cut. The G.I.A. report does not provide this information. To obtain the crown angle and the pavilion depth/angle measurements, you need either a Sarin or Mega-Scope report (in addition to the G.I.A. Report). As noted, a G.I.A. Report and a Sarin Report, (noting the crown and pavilion depth/angle, etc.), are available for all diamonds sold by Dimonz.

It should be noted that in assigning cut grades, the extremity sets the grade-- that is, if a single cut characteristic (e.g., crown angle, table %, etc.) falls into a lower category, the diamond is assigned the cut grade for that category. For example, if a diamond meets all of the criteria for G.I.A.'s Class I cut grade but has a 62% table (a Class II characteristic) the diamond is assigned a Class II cut grade.

After G.I.A.'s four cut grades are defined and terms pertaining to cut are explained, there is a thorough discussion of what is meant by "Ideal Cut." Please note that the vast majority of the diamonds sold in the U.S. Market which are purported to be "Ideal Cut" diamonds, do not meet the criteria for this cut classification.
 

G.I.A. CUT GRADES

CLASS I (the top cut grade, resembling the "Ideal Cut"): have a 53%-60% table; "Very Good" or "Excellent" Polish and Symmetry; a 34-35 degree crown angle; and a 43% pavilion depth (that is, the depth of the pavilion or bottom part of the diamond is 43% of it's width when rounded to the nearest 1/2%). In addition, the diamond must meet the other cut criteria described, below. You generally pay 15% more for diamonds with a Class I cut grade than for a similar diamond with a Class II cut grade.

CLASS II ("International Fine Trade Cut"): have a 61%-64% tables; a 32-34 degree crown angle; a 42.0%-42.5% or 43.5%-44.0% pavilion depth (when rounded to the nearest 1/2%); at least "Good" Polish and Symmetry; in addition to meeting the other criteria described below. These are generally regarded in the trade as well cut stones. Personally, I will only buy a Class II diamond with a table of 62% or less. In my opinion, a 63% or 64% table represents the lower end of this cut grade and seems to be the point at which you start to observe a significant reduction of fire.

CLASS III ("U.S. Domestic Average Cut") Diamonds generally have a 65%-70% table; a 30-32 degree crown angle; a 41%-41.5% or 44.5%-46% pavilion depth; and "Fair" Polish and Symmetry. Other measurements may also fall outside of those prescribed above. These diamonds typically sell for 10% to 30% less than those with a Class II cut grade.

CLASS IV: ("Below Average Beauty/Durability) typically discount 30% to 50% or more from those with a Class II cut grade.

IMPORTANT TERMS RELATING TO CUT (USED IN THE DISCUSSION OF "IDEAL CUT" DIAMONDS, BELOW):

Diagram A

1. TABLE PERCENTAGE- This 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 diamond, i.e., the "girdle diameter" (see Diagram A). For example, if a diamond with a 6.50 millimeter diameter (i.e., the girdle diameter) has a table measuring 3.90 millimeters across, 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 (unless the table is under 52.5%, which is rare). Generally speaking, well cut diamonds should have a table percentage of 53%-63%. Most gemological institutions allow for a table of between 53%-58% for "Ideal Cut" diamonds (e.g., the American Gem Society specifies a table percentage of 52.5%-57.5%), with some allowing up to 60% (e.g., G.I.A.'s Class I cut).

Why are small tables desirable? There are two types of brilliance which come from a diamond: "life" and "fire" (see diagram B). The "life" of the diamond is the white light that is reflected from the table. The "fire" of the diamond is the multicolored (blue, red, green, orange, yellow and violet), scintillating light that reflects through the angled part of the crown of the diamond. The "fire" is regarded as being more beautiful than the "life" or white light. Stated simply, smaller tables are desirable because, diamonds with small tables 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 diamond rough (note: since diamonds are sold by carat weight, manufacturers generally make more money this way, although the play of light in the diamond is negatively affected). You generally pay more for a diamond with a smaller table because of the greater weight loss from the rough diamond.

Diagram B

2. GIRDLE (see Diagram C)- This factor measures the thickness of the "girdle" or the outside circumference of the diamond. There are typically two measurements given on a G.I.A. Report. 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 one of the girdle measurements -- it may be prone to chipping along the girdle. Don't buy a diamond that has " very thick" or "extremely thick" as one of the girdle measurements-- the cutter has added unnecessary weight to the diamond which you will pay for! Also don't buy or those with a single measurement of "thick." Again, the cutter has added unnecessary weight to the diamond. Generally, the following girdle thicknesses are acceptable for all cut grades, including "ideal cut" stones : 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.

3. CULET (Diagram D) - The culet is a very small facet on the pointed end of the diamond. The following are acceptable for all classes of cut, including Class I or "Ideal": 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
 

4. POLISH- This element measures the quality of the finish on the surface of the diamond, e.g., the presence of abrasions, polishing lines and other surface characteristics that were made when the diamond was cut. There are five grades- i.e., Poor, Fair, Good, Very Good, Excellent. Any diamond you consider purchasing should have a polish grade of "Good" or better. Class I or "Ideal" cut diamonds should have a polish grade of "Very Good" or "Excellent." However, please be aware that the Polish grade is not nearly as important as other characteristics (e.g., table percentage, crown angle and pavilion depth) in determining the optical efficiency of a diamond. Polish and Symmetry when considered together, comprise only 1/5 of the cut grade of a diamond.

5. SYMMETRY- This element measures the shape and overall cutting detail of the facets. There are five grades- i.e., Poor, Fair, Good, Very Good, Excellent. As previously noted, the symmetry grade in combination with any diamond you consider purchasing should have a symmetry grade of "Good" or better. Class I or "Ideal" cut diamonds should have a polish grade of "Very Good" or "Excellent."

6. 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 multi-colored "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.

7. DEPTH (see Diagram E) - On a G.I.A. Certificate, this figure represents the depth of the diamond expressed as a percentage of it's width. It is one of the most important measurements. To get the "depth percentage", you take the depth of the stone in millimeters (the last figure under measurements) and divide by the diameter (i.e., the Average Girdle Diameter). Generally speaking, the Depth Percentage should be between 58.5% and 62.8%. Shallow stones with a depth percentage of less than 58% are called "spready makes" in the gem trade (Diagram E, far right figure). They appear to be larger than normally cut stones of the same weight but they tend to be less brilliant. Stones with a depth of over 64% are termed "heavy" or "deep makes" (Diagram E, 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 (called "nailheads" in the gem trade). "Heavy makes" 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.
 

Diagram E

8. PAVILION DEPTH (not recorded on the G.I.A. report) -- 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 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 E, 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%).

9. PAVILION ANGLE (not recorded on the G.I.A. report) -- is an angle which correlates directly to the pavilion depth. A pavilion depth of 42% generally equates to a pavilion angle of about 40.0 degrees; a pavilion depth of 42.5% generally equates to a pavilion angle of about 40.5 degrees; a pavilion depth of 43% generally equates to a pavilion angle of about 40.7 degrees; a pavilion depth of 43.5 degrees generally equates to a pavilion angle of about 41.0 degrees; a pavilion depth of 44% generally equates to a pavilion angle of about 41.3 degrees; a pavilion depth of 45% generally equates to a pavilion angle of about 42.2 degrees. 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.

How important is each of these factors in determining the brilliance of a diamond? Below, I've done something very unscientific. Assuming we stay within Class I or Class II cut parameters (as defined, above) how important is each factor on a scale from 0 to 10 in determining the optical efficiency of a diamond?-- 0 = not important at all; 1-3 = moderately important; 4-7 = important; 8-10 = (critical; extremely important).

Table Percentage......................6
Polish......................................2
Symmetry...............................2
Crown Angle............................8
Pavilion Depth/Angle..............10
Girdle/Thickness/Culet/Depth must be within guidelines described, above.

DISCUSSION OF "IDEAL CUT" DIAMONDS

The cut grade of a round brilliant diamond is generally based on how closely it conforms to the "American Ideal Cut". The Ideal Cut was developed by a mathematician named Tolkowsky in 1919. The purpose of the Ideal Cut was to to produce the most brilliant diamond possible by maximizing light return.

There are some diamond manufacturers who specialize in fine cut stones that closely resemble Tolkowsky's model and have excellent cutting detail. "Super-Ideal" cut diamonds (which cut very closely to Tolkowsky's original model, and have superior cutting detail) and "Hearts and Arrows" diamonds ("ideal cuts" which produce a distinctive hearts and arrows image when viewed through a special device) are examples, as are diamonds cut by Lazare Kaplan (a firm specializing in ideal cuts). However, most of these diamonds are very expensive, costing up to 40% or more than diamonds available on the open market which are basically equivalent in terms of optical efficiency.

There are also many variations of Tolkowsky's ideal cut which employ the same basic faceting arrangement (i.e., 59 facets) and have similar proportions, such as the "Eppler Fine Cut"; the"Scandinavian D.N."; and the "Johnson and Rosch" model (note: this model differs significantly from Tolkowsky's in it's crown and pavilion angles).

In addition there are specialty cut round stones which generally have more facets in a different arrangement and are purported to be more optically efficient than the Tolkowsky's Ideal Cut, e.g., the "Jubilee Cut" (80 facets), the "King Cut"(86 facets) , the "Magna Cut"(102 facets), and the "Royal 144" (144 facets). However, round diamonds with one of these specialty cuts are generally difficult to find at workable prices.

All of this having been said, even in the information age, the "American Ideal Cut" developed by Tolkowsky remains one of the most optically efficient models for diamonds ever developed. Generally speaking, it is also the benchmark upon which the cut of all round brilliant diamonds are currently judged. The measurements of Tolkowsky's model are as follows:

TOLKOWSKY'S "AMERICAN IDEAL CUT"
Table: 53%
Crown Angle: 34.5 degrees
Pavilion Depth: 43.1%
Polish: (Unspecified)
Symmetry: (Unspecified)
Girdle: (Unspecified)
Culet: (Unspecified)

How closely must a diamond conform to Tolkowsky's original model to be considered "ideal"? In America, there are two widely accepted schools of thought on this issue. The Gemological Institute of America's (G.I.A.'s) "Class I" cut and the American Gem Society(A.G.S.) Ideal Cut are similar, but differ in some respects.

The American Gem Society (A.G.S.), specifies the following parameters for Ideal Cut diamonds, which they assign a cut grade of "0" on their diamond reports (on a scale of 0 to 10):

A.G.S. IDEAL CUT
Table: 52.4-57.5%
Crown Angle: 33.7-35.8 degrees
Pavilion Depth: 42.2%-43.8%
Polish: Ideal*
Symmetry: Ideal*
Girdle: Thin, Medium, Slightly Thick
Culet: None, Very Small, Small, Medium

*A.G.S. "Ideal" is roughly equivalent to G.I.A.'s Very Good or Excellent grades for these factors.

The Gemological Institute of America (G.I.A.) does not recognize an ideal cut diamond per se. However, G.I.A.'s Class I cut closely resembles the "Ideal" proportions defined by Tolkowsky:

G.I.A. CLASS I CUT
Table: 53-60%
Crown Angle: 34-35 degrees
Pavilion Depth: 43% (i.e.,42.75%-43.25%)
Polish: Very Good to Excellent
Symmetry: Very Good to Excellent
Girdle: Medium to Slightly thick
Culet: None, Very Small, Small, Medium

Please note the differences between the A.G.S. Ideal Cut parameters and the G.I.A. Class I cut parameters. The A.G.S. is stricter in terms of the acceptable Table percentage for an Ideal Cut diamond, allowing a maximum of 57.5%, while a G.I.A. Class I cut allows for up to a 60% table.

G.I.A. is stricter in terms of the acceptable Crown Angle (which affects the quality of dispersion or fire in the diamond) and Pavilion Depth (which determines how much of the light which enters a diamond is reflected to the eye as "brilliance"). G.I.A. allows for crown angles from 34-35 degrees while A.G.S. allows from 33.7-35.8 degrees; G.I.A allows for Pavilion Depths of 42.75-43.25% while A.G.S. allows from 42.2%-43.8%.

Who's right, A.G.S. or G.I.A.?.... I don't know. Generally speaking, diamonds with either G.I.A.'s Class I cut or an A.G.S. "0" Ideal Cut are magnificently brilliant and there don't appear to be significant differences in light return. The bottom line is that you'll generally pay 15% more for either one than you will for a stone that is simply "well-cut" (e.g., A Class II cut).

OTHER VIEWS OF "IDEAL CUT" DIAMONDS

To further complicate this discussion, there are several other gemological organizations which have defined their own version of what constitutes an "ideal cut." For example, an organization known as the Accredited Gem Appraisers (U.S.A.) has an excellent system used by most American gem appraisers, which breaks down each of G.I.A.'s cut grades (Classes I-IV) into two subclasses, using a formula that is to complex for the lay person.

The International Gemological Institute stipulates that "Ideal Cut" stones have a 53%-60% table; a 33-36 degree crown angle; a 42%-44% pavilion depth; a thin to slightly thick girdle; and a total depth of 58.5%-62.5%. The European Gemological Laboratory's top cut grade is "Exceptional" and is characterized by a 54%-57% table; a 34-35 degree crown angle; a 42.5%-43.5% pavilion depth; and a thin to medium girdle (total depth is unspecified).

The top cut grade recognized by the the Central Gem Laboratory (C.G.L.) of Japan have a 52%-58% table; a 33-35 degree crown angle; a 42%-44% pavilion depth; a thin to slightly thick girdle; and a total depth of 59.2%-62.4%. The requirements for assigning an "Excellent" cut grade specified by the Association of Gemological Laboratories (A.G.L.) of Japan are identical to those of the CGL with the exception of allowable table % (C.G.L. allows for 52%-58% vis a vis 53%-58% for A.G.L.). The top grade recognized by the Association of Japan Gem Trust has a 53%-60% table; a 33-35 degree crown angle; a 43% pavilion depth; a thin to slightly thick girdle; and a total depth of 59%-63%.

BOTTOM LINE: So whose definition of ideal cut do we accept? The current thinking is that both the A.G.S. "Ideal Cut" (as defined above) and G.I.A. Class I Cut, are considered the norms for the best cut diamonds available in the market. Diamonds in these cut categories comprise approximately 1%-3% of the total number of stones available in the diamond market. For either the A.G.S. or G.I.A. model, you will pay approximately 15% more than you will for G.I.A.'s Class II (well-made or "International Fine Trade Cut") diamond (see definition, above).

THE GENUINE IDEAL CUT DIAMOND-- HOW RARE??

The most complete study of the optical efficiency of round diamonds was published in the Fall 1998 edition of G.I.A.'s publication, Gems & Gemology (titled: "Modeling the Appearance of the Round Brilliant Cut; An Analysis of Brilliance"). One of the interesting facts pointed out in this study, is that of the 67,621 diamonds analyzed, only 2,051 or 3% had proportions that fell into the "ideal" range, which they defined for purposes of this study as: a 53%-57% table; a 34.0%-34.9% crown angle; and a 42.5%-44% pavilion depth* which converts to a40.2-41.3 degree pavilion angle.

How many of the 3% of diamonds with ideal proportions meet the other criteria for an Ideal Cut diamond, i.e. proper girdle thickness and Very Good to Excellent on Polish and Symmetry grades? We're probably talking about 1% of available diamonds that meet the "Ideal Cut" criteria.

If you want to purchase an Ideal Cut diamond, does this mean that 1% of the diamonds in your weight category (e.g. 1 carat, 2 carat, etc.) are worth considering? -- No. In actuality you may be looking at as few as 1/5th of 1% (one in 500 diamonds of a particular carat weight) to 1/100th of 1% (one in 10,000) when you factor in other variables. First of all, you are probably interested in considering diamonds only in a particular color and clarity range, e.g., G or H in color and VS2 to SI1 in clarity. You may also want to eliminate from consideration diamonds that have strong or very strong fluorescence. Most importantly, you probably want to eliminate at least 75% of those remaining on the basis of cost. Remember, because of their scarcity and the increasing demand for Ideal Cut diamonds, finding a good deal on one is often very difficult.

* (Note: these percentages fall outside of the range defined for G.I.A.'s Class I and the A.G.S. Ideal Cut).

When you factor in all of these parameters, the ideal cut diamond you seek usually comprises an infinitesimal portion of the market. My job is to help you find it at the lowest price available anywhere!

WHY IS IT SO IMPORTANT TO HAVE A SARIN REPORT IN ADDITION TO THE G.I.A. REPORT?

Let's illustrate with an example. You are in the market for an ideal cut, G VS2 diamond, weighing about one and a half carats. You think you've found one that sells for 10% lower than any other one you've seen. The G.I.A. Report for the diamond reads as follows:

Measurements: 7.42-7.45 X 4.54 mm
Weight: 1.50 carats
Depth: 61.1%
Table: 57%
Girdle: Medium
Culet: None
Polish: Excellent
Symmetry: Excellent
Clarity: G
Color: VS2
Fluorescence: None
Comments: None

Let's analyze this report. The Measurements are fine for a diamond weighing 1.50 carats. The Depth falls easily between 58% and 62.5%. The 57 % Table meets the criteria for both the A.G.S. Ideal Cut and the G.I.A. Class I cut grade. It has a medium Girdle thickness, no Culet -- fine so far. It also has Excellent Polish and Symmetry. There is no Fluorescence and the Comments section is blank. Why is this "ideal cut" diamond selling so cheap?

The answer is that this diamond is not ideal cut. In fact it is cut very poorly. If a Sarin report were available, it would indicate that the diamond has a shallow crown , producing a crown angle of 30 degrees (negatively affecting the fire in the stone). Additionally, the pavilion depth is 46%, i.e., the pavilion is too deep. This means that most of the light that enters the diamond falls through the bottom part of the stone rather than being reflected back to the eye, creating a dark, lifeless diamond (as in the middle figure in Diagram E, above).

What looks like a great deal on a Class I or Ideal Cut diamond, is really a lousy deal on a Class III - Class IV diamond that would typically sell for about 30% -45% less.

Why would a diamond be cut this way? Why wouldn't the cutter give this diamond a Class I or Ideal Cut? The answer is simple. Diamonds are sold by weight. Cutters lose weight from the diamond rough when they cut a Class I or Ideal Cut stone. The same rough diamond that produced this 1.50 carat Class III cut diamond might have only produced a 1.25 carat Class I cut diamond (which is would sell for $1,500-$2,000 less).

HOW DO I GET THE BEST CUT DIAMOND FOR THE LEAST AMOUNT OF MONEY?

In my opinion, one of the best ways to save money and get an extraordinarily optically efficient diamond, is to buy a diamond which meets all of the criteria for the A.G.S. Ideal Cut but barely misses on one or two factors - for example, a diamond which has a 33.5 degree crown angle ( A.G.S. "ideal" is 33.7-35.8 degrees) or a 57.6%-60% table (A.G.S. "ideal" is 52.4%-57.5% with G.I.A. aloowing up to 60% for it's Class I cut) , and/or receives a "Good" instead of "Very Good" on Polish or Symmetry. For all intents and purposes, these diamonds are as optically efficient as an ideal cut stones but cost less.

 Index

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Aggiornato il: 15/06/2006