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The 4 Cs of Diamonds: Cut

Diamond 4 Cs Cut

The 4 C's Diamond Grading System

More than 100 million diamonds are sold in the United States each year, yet most consumers know very little about the product they are purchasing, and how that product is valued. The '4 Cs' represent the four main variables that are used to calculate the quality and value of a diamond. Both rough and cut diamonds are separated and graded based on these four characteristics.

As a consumer, your first step in shopping for a diamond should be to learn and understand the '4 Cs' diamond grading system. If you are purchasing an expensive stone it will also be critical for you to learn how to read and understand the details of a GIA (Gemological Institute of America) 'Diamond Dossier,' AGL report, or AGS (American Gem Society) 'Diamond Certificate,' or Sarin 'Diamond Grading Report' (see full list of independent testing laboratories, below). You will also want to familiarize yourself with the Federal Trade Commission (FTC) guidelines on jeweler conduct and consumer awareness. This knowledge will help be invaluable when you are comparison shopping for diamonds.


Diamond Cut Quality

When jewelers judge the quality of a diamond cut, or "make", they often rate "Cut" as the most important of the "4 Cs." The way a diamond is cut is primarily dependent upon the original shape of the rough stone, location of the inclusions and flaws to be eliminated, the preservation of the weight, and the popularity of certain shapes. Don't confuse a diamond's "cut" with it's "shape". Shape refers only to the outward appearance of the diamond (Fig. 5 below), and not how it is faceted.

The Importance of Cut Quality

When a diamond has a high quality cut (ideal cut), incident light will enter the stone through the table and crown, traveling toward the pavilion where it reflects from one side to the other before bouncing back out of the diamond's table toward the observer's eye (see Fig. 1 below). This phenomenon is referred to as "light return" (Fig. 2 below) which affects a diamond's brightness, brilliance, and dispersion. Any light-leakage caused by poor symmetry and/or cut proportions (off-make) will adversely affect the quality of light return.

The "Shallow Cut" and "Deep Cut" examples in Fig. 1 show how light that enters through the table of a Modern Round Brilliant diamond reaches the pavilion facets and then leaks out from the sides or bottom of the diamond rather than reflecting back to the eye through the table. Less light reflected back to the eye means less "Brilliance". In the "Ideal Cut" example, most of the light entering through the table is reflected back towards the observer from the pavilion facets.

Fig. 1

Four Cs Diamond Cut Quality

Keep in mind that the variance in proportions between an "Ideal Cut" (ideal make) and a "Fair, Poor, Shallow or Deep Cut" may be difficult to discern to the novice observer, although there will be a lack of brilliance, scintillation, and fire. Cut quality is divided into several grades listed below.

  • Ideal Cut
  • Premium Cut
  • Very Good / Fine Cut
  • Good Cut
  • Fair Cut
  • Poor Cut

Cut Proportions

In the past, the "Cut" quality of the "4 Cs" was the most difficult part for a consumer to understand when selecting a good diamond because a GIA or AGS certificate did not show the important measurements influencing cut (i.e. pavilion and crown angle) and did not provide a subjective ranking of how good the cut was. Only a trained eye could see the quality of a good cut. All of that has changed with the AGS Cut Grading system and GIA's new "Cut Grading System".

Fig. 2

Faceting a Gemstone - Brilliance & Light Return

The proportion and symmetry of the cuts as well as the quality of the polish are factors in determining the overall quality of the cut. A poorly cut diamond with facets cut just a few degrees from the optimal ratio will result in a stone that lacks gemmy quality because the "brilliance" and "fire" of a diamond largely depends on the angle of the facets in relation to each other. An Ideal Cut or Premium Cut "Round Brilliant" diamond has the following basic proportions according to the AGS:

  • Table Size: 53% to 60% of the diameter
  • Depth: 58% to 63% of diameter
  • Crown Angle: 34 to 35 degrees
  • Girdle Thickness: medium to slightly thick
  • Facets: 58 (57 if the culet is excluded)
  • Polish & Symmetry: very good to excellent

The girdle on a Modern Round Brilliant can have 32, 64, 80, or 96 facets which are not counted in the total number of facets (58). The crown will have 33 facets, and the pavillion will have 25 facets. Other variations of the "Modern Round Brilliant" include the "Ideal Brilliant" which was invented by Johnson and Roesch in 1929, the "Parker Brilliant" invented in 1951, and the "Eulitz Brilliant" invented in 1972.

Poor Diamond Faceting and Symmetry

Due to the mathmatics involved in light refraction, a Round Brilliant cut that does not have the proper proportions and symmetry (off-make) will have noticeably less brilliance. Common cutting problems can occur during the faceting process, when one incorrect facet angle can throw off the symmetry of the entire stone. This can also result in the undesirable creation of extra facets beyond the required 58. The chart below shows several common problems to look for.

Fig. 3

Poor Faceting and Symmetry

For a Modern Round Brilliant cut (Tolkowsky Brilliant), there is a balance between "brilliance" and "fire". A diamond cut for too much fire will look like cubic zirconia, which gives out much more fire than a real diamond. A well executed round brilliant cut should reflect the maximum amount light from the interior pavilion facets, out through the table, making the diamond appear white when viewed from the top. A cut with inferior proportions will produce a stone that appears dark at the center (due to light leaking out of the pavilion) and in some extreme cases the ring settings may show through the top of the diamond as shadows.

GIA vs AGS Cut Grading

GIA's new cut-grading system is based on averages that are rounded-up to predict 'light performance,' while AGS uses a more exacting combination of proportional facet ratios along with ray-tracing metrics to calculate light return. The "Ideal" designation is an AGS term that is not found on an GIA report. The GIA will give a symmetry demerit for what it calls "non-standard brillianteering" which some manufacturers use to 'improve' on the standardized Tolkowsky-type cuts.

AGS Triple-0 Certification

The American Gem Society (AGS) is the industry leader in laboratory testing of round gems for cut grade and quality. In order for a diamond to receive a "Triple-0" grading, all three categories of cut (Polish, Symetry, Proportion) must meet the "ideal" criteria. A Triple-0 diamond can also be called a "Triple Ideal Cut" or "AGS-Ideal Zero" diamond.

Hearts and Arrows Diamonds

A perfectly proportioned ideal cut that is cut to the exacting specifications of a Tolkowsky Cut, Eppler Cut (European Standard), or a Scan D. N. Cut (Scandinavian Standard) will display a "Hearts and Arrows" pattern when observed through a IdealScope (arrows only), or a H & A Viewer gemscope (FireScope).

Fig. 4

Hearts and Arrows Diamonds

Perfectly formed Hearts and Arrows patterns with eight hearts AND eight arrows (above, left) are only found in diamonds that meet the American Gem Society Laboratories' "0" Ideal Cut specifications. The IdealScope was invented by Kazumi Okuda in the 1970's, and its later incarnation, the "FireScope," was invented by Ken Shigetomi and Kazumi Okuda in 1984. The first official H & A "EightStar" diamond was cut in 1985 by Kioyishi Higuchi for Japanese businessman and FireScope manufacturer, Takanori Tamura.

Fancy Diamond Cuts

The shape of the cut is a matter of personal taste and preference. However, the quality of the cutter's execution of that shape is of primary importance. The shape of the diamond cut is heavily dependent upon the original shape of the rough stone. The round brilliant cut is preferred when the crystal is an octahedron, as two stones could be cut from one crystal. Asymmetrical raw crystals such as macles are usually cut in a "Fancy" style. Several basic diamond shapes (Fig. 5) are listed below.

  • Emerald
  • Heart
  • Marquise
  • Oval
  • Pear
  • Princess
  • Radiant
  • Round
  • Trillion (not shown at diagram)

Fig. 5

Diamond Cuts

Popular fancy cuts include the "Baguette" (bread loaf), "Marquise" or "Navette" (little boat), "Princess" (square outline), "Heart," "Briolette" (a form of Rose cut), and the Pear. The "fancy cuts" are generally not held to the same strict standards as Round Brilliants.

On To:

Independent Diamond Testing Laboratories

Patented Signature Diamond Cuts

Old European Diamond Cuts

Fancy Diamond Cuts

Diamond Cutting

Diamond Chemistry & Composition

Diamond Books
Diamond Books

Bibliography & Suggestions for Further Study on Diamonds

1. Fred Cuellar, How To Buy A Diamond 5th Edition . Sourcebooks Casablanca

2. HCA IdealScope, Hearts and Arrows .

3. Marcel Tolkowsky, Diamond Design .

4. Sarin, Diamond Grading . Sarin Gem Labs

5. AGS, American Gem Society - Diamond Grading .

6. GIA, Gemological Institute of America .

7. Federal Trade Commission (FTC), In The Loupe: Advertising Diamonds .

8. Federal Trade Commission (FTC), All That Glitters... How to Buy Diamonds .

9. Joseph Mirsky, Consumer Guide To Diamonds . Joseph's Jewelry

10. George E. Harlow, The Nature of Diamonds . Cambridge University Press

11. Judith Crowe, The Jeweler's Directory of Gemstones . DK Publishing.

12. Renee Newman, Gemstone Buying Guide . International Jewelry Publications; 2nd edition

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