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Cut Diamond Characteristics: Cut


Cut alone is the single most important factor in determining the fire and brilliance of a diamond. The precision of each cut is vital because the more a diamond reflects and refracts light, the better its radiance and intensity.

A poorly cut diamond, cut too deeply or too shallow,

   

will allow light to escape through the opposite side or through the bottom of the stone before it can reflect light back to the surface. A well-cut diamond, with the correct amount of facets and precision angles in good proportions, will allow the light to bounce freely inside--reflecting from one facet to another until it emerges from the top with all its promised brilliance. A well- cut diamond enhances all of the other characteristics of the stone.

The awe-striking beauty of a cut diamond emanates from its inherent physical and optical properties in combination with a cutter's ability to maximize the display of these properties. It is the precision, care and artistry, which the cutter brings to the stone that releases the diamond's full potential. In purchasing a diamond one must consider two elements of cutting, which are of prime importance - proportion and finish (which is further sub-divided into two categories, polish and symmetry).    

 

Here we focus on the modern round brilliant cut diamond because it is the most popular shape sold and it produces the best light return, in the form of brilliance, dispersion and scintillation, to the eye. (Please read our article on the Fancy Shaped Diamonds for more detailed information on oval, pear, marquise, radiant, princess, heart and emerald cut diamonds.)

Brilliance is the total light reflected to the eye from both surface and internal reflections.

 

   

Dispersion is the separation of white light into its spectral (rainbow) colors and is caused by its refraction, which bends each wavelength of light a little differently to cause separation into individual colors.  

   

Scintillation is the flashing or sparkling of light which occurs when either the diamond or light source are moved.

 

   

To easily understand what goes into evaluating the cut proportion and finish of a standard round brilliant (SRB) diamond, let's first take a look at a diagram showing each element of the finished product (below). Down the left-hand side are listed the major structural features of the diamond, and down the right-hand side the different facet names.

 

   

There are a total of 58 facets, called the table (1) and culet (1), bezel (8), star (8), upper girdle (16), lower girdle (16) and pavilion (8) facets. Often there is no culet facet and the stone is pointed on the bottom, leaving a total of 57 facets. Also, in higher quality goods the girdle is frequently faceted, but these facets are not counted in the total.

Finish, which includes polish and symmetry, is also an important contributor to a diamond's beauty.

Diamond with chipped gridle affects "polish."

Polish features largely describe the condition of the surface of the stone, and include abrasions of the facet junctions and the culet, rough and bearded girdles, nicks and pits, and scratches and polishing marks if they do not affect the clarity grade. You will remember some these elements were also listed as blemishes in the clarity-grading scheme. However, if the blemishes are not serious enough to affect the clarity grade, they are included under polish as part of the finish grade.

Symmetry characteristics include many features of poor and/or careless workmanship such as an off-center table or culet, a table not a regular octagon shape, an out-of-round or wavy girdle, non-parallel girdle and table, facets which don't meet or point properly or are misaligned between the crown and pavilion, and naturals and extra facets which are not graded under clarity.

Left: Table cut large, less reflection; Right: talbe cut smaller, more light reflected upward

   

Proportion - The full realization of the potential of a diamond's brilliance, dispersion and scintillation and getting the proper balance between these three elements is achieved mainly through proper cut proportions. The important proportions are:
1. Table size
2. Crown height and angle (angle of the bezel facets with the girdle)
3. Pavilion depth and angle (angle of the pavilion facets with the girdle)
4. Girdle thickness
5. Culet size

As shown in the photo below, if a round brilliant diamond is cut too deep (nailhead) or too shallow (fisheye) light leaks out of the pavilion and does not return to the observer's eye. In diamonds with too steep a crown angle (>38deg.), exiting light is directed out to the side where it isn't visible, or it can be reflected back into the stone. In diamonds with large tables, if the crown angle is too shallow (<30deg.) there is little visible dispersion of light.

 

Fisheye from too shallow a cut causes loss of light to top and diminished brilliance.

   
Make
 

- This is a trade term used to describe how closely the proportions of a brilliant cut diamond are to an "ideal". For stones approaching these proportions a diamond is said to be of good or fine make, and for those with problems of proportion are referred to as being of fair or poor make. While there are different sets of criteria (Yes, the definition of Ideal can vary.) for what constitutes an Ideal cut in a round brilliant diamond, there are presently none for the other fancy shaped cuts (oval, marquise, emerald, radiant, princess, heart, pear). (See www.gemappraisers.com for the best classification of cut proportions in fancy shapes currently available.)

An Ideal Cut round brilliant diamond( see tolerances below) is one which is cut to a specific range of proportions (Ideal proportions) and also has an Ideal (0) grade for both polish and symmetry. This has been defined by the American Gem Society Labs. as the top, or zero grade, in their grading system. Diamonds attaining this grade are sometimes referred to as AGS000 (AGS triple zero's). The proper grade, however, is simply AGS0 (AGS zero). This cut grade has also come to be called the "American Ideal Cut" based on its acceptance in this country.

 

Unfortunately, the Gemological Institute of America (GIA) does not provide a cut grade in its grading report. They provide only the table % and total depth % for the stone. Therefore, it is not possible to determine the individual contributions of the crown, girdle and pavilion to the total depth and no crown or pavilion angles are given. In order to determine if a GIA-graded diamond meets the AGS Ideal (0) grade for proportion, it is necessary to measure the stone for all the missing data. Fortunately, this can be done quite quickly and easily using an automated proportion analyzer called a "SARIN" (after the Co. which produces it) whose software can be programmed to AGS specifications. Thus, since GIA grades of Excellent for polish and symmetry are considered equivalent to the AGS Ideal grades, a GIA-graded diamond can, with a Sarin analysis give as much information as an AGS grading report. This information is important in making an informed buying decision.

Effect of cut quality on price: Cut quality commands a premium for two reasons. First, you are paying for a highly skilled diamond cutter's time, and it can take many hours to get all the proportions and angles to fall within the Ideal or SuperIdeal ranges. Second, more of the diamond is lost in the cutting process, because the goal is not to cut the heaviest diamond, but the best performing diamond.

Quantifying the premium paid for an ideal cut diamond is difficult and varies greatly depending upon the source, e.g., Tiffany, Lazare Kaplan, AGS0's, branded and non-branded Hearts & Arrows. For internet-based dealers, whether or not they also have a bricks and mortar operation, the premium is very roughly around 10-15% over a poorly cut stone.

A final point about GIA and AGS grading reports. Diamonds graded by these labs. carry about an 8% premium over those graded by other labs. such as EGL, IGI, HRD, etc. The reason is due to their strict grading guidelines, which are respected and accepted worldwide. You can rest assured that if your diamond is graded by either GIA or AGS that the color, clarity, polish and symmetry are what the report says, so you know what you are paying for. The premium is well worth it!

 

 

 

 
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