With the recent introduction of the Cut grade by both the GIA (Gemological Institute of America) and the AGS (American Gemological Laboratories) labs the least understood and most important of the 4C's has been standardized. Both the GIA and the AGS labs have different approaches to grading the cut of a diamond, but both are scientifically-based proven systems. The GIA bases its cut grade on proportions and the AGS bases its cut grade on light performance. GIA uses a scale with 5 levels (excellent to poor) where the AGS system has 11 levels (0 to 10). The Cut grade is the biggest change in the diamond industry in the last 25 year.

A highly trained diamond cutter must first analyze the rough diamond before considering what cut will complement the natural beauty of the stone. Interestingly, a diamond can only be cut by another diamond.

How Important is the Cut of a Diamond?

It is possible to have two diamonds that are equal in carat size, share the same color and clarity but look completely different. It all depends on the cut. Based on the cut, the weight of the diamond can be distributed differently to various parts of the diamond such as the crown and girdle. Also, the way facets are arranged and how brilliant the diamond appears will be determined by the way the diamond is cut.

Diamond Cut Grading

In 2005, the GIA introduced its 'Diamond Cut Grading System' for round brilliant diamonds. This system has been in development for fifteen years and the researchers at GIA have been perfecting a computer modeling system for round brilliants. They experimented on more than 70,000 stones in order to successfully establish this new grading system. The result of the research is that the system can now predict the cut grade for millions of various sets of round brilliant diamonds. This new 'Diamond Cut Grading System' analyzes the diamond's face-up appearance by using many different computer modeling techniques in addition to algorithms, which are used to evaluate the levels of brightness, fire and scintillation.

The science supporting the AGS Lab's cut grade system is the result of years of research, testing, and retesting by a team of gemologists, optical physicists, and mathematicians. In the AGS cut grade system, all facets of a diamond are measured in three dimensions rather than two dimensions. The AGS Lab uses a state-of-the-art ray tracing software to trace light traveling through a diamond. The software demonstrates the quantity and quality of the light being returned to the viewer. AGS Laboratories will now consider brightness, dispersion, leakage, contrast, and weight ratio, in addition to proportion, symmetry, and polish, when assigning a final cut grade.

Scale
GIA	Excellent		Very Good		Good		Fair			Poor
AGS	0	1		2	3	4	5	6	7		8	9	10

Since GIA and AGS Labs use different scales and methods, this represents DiamondWaves opinion as to the equivalence of the two grading systems.

GIA, AGS and the New Cut Grading: The Metrics

The following is a summary of individual metrics used by each lab for cut grading.

GIA Metrics

The GIA Cut scale levels are: Excellent, Very Good, Good, Fair and Poor. The GIA Cut grade assesses the diamonds in these primary ways:

Face-Up Appearance is the most important part of the GIA's cut grade assessment.

• Brightness: Usually referring to the "brilliance" of a stone.
• Fire: Display of colored light return as observed in stronger lighting conditions.
• Scintillation: Movement of the light's pattern within the diamond as the diamond observer or light source is moved.

Diamond Physical Design

• Weight Ratio: Refers to the appearance of the size of the diamond in relation to its weight. i.e., diamonds cut too deep appear to be smaller than their intended weight.
• Durability: Refers to the possibility of the diamond chipping or breaking due to extremely thin girdles. GIA did not find evidence that a diamond's durability suffered from shallow angles, unless the girdle was very thin.

Diamond Craftsmanship

• Polish: Quality of the finish ultimately affecting luster.
• Symmetry: Facet placement and their alignment with adjoining and opposite corresponding facets.

AGS Metrics

The AGS Cut scale levels runs from 0 for 'Ideal Cut' to 10 for 'Poor'. Because the AGS bases their cut grade on light performance this allows them to grade for shapes other than round. The AGS Cut grade assesses the diamonds in these primary ways:

Light Performance

• Brightness: Usually referring to the "brilliance" of a stone
• Dispersion: The appearance of colored light return through the crown in conditions that enhance the metric.
• Leakage: Facets which are functioning in a role of "window" rather than "mirror" or "non-reflector" as opposed to a "reflector".
• Contrast: The light and dark patterns seen when observing a diamond. It can produce a positive or negative optical effect. This usually is caused by, but not limited to, the observer's head.

Diamond Proportion Factors

• Tilt: Not only is the face-up view considered but tilting the stone to examine its optics is considered.
• Durability: Refers to the possibility of the diamond chipping or breaking due to extremely thin girdles.
• Weight Ratio: Refers to the appearance of the size of the diamond in relation to its weight. i.e., Diamonds cut too deep appear to be smaller than their intended weight.
• Girdle thickness: too thin or too thick is not considered desirable
• Culet Size: must not be chipped or too large.

Diamond Finish

• Polish: Quality of the finish ultimately affecting luster
• Symmetry: Display of colored light return as observed in stronger lighting conditions

Optical Characteristics/Metrics

The pptical characteristics of a diamond determining light performance and appearance.

• Contrast: If a stone returns light so that the crown is bright all over, it would not appear as bright to your eye as one with good contrast. While light return is an important part of 'brilliance', our perception of brightness is enhanced by our eyes adaptating to contrasting backgrounds. Diamonds with good contrast, caused by alternating very bright and very dark facets, appear brighter because of perception and physiology of the way the eye works.
• Brightness: Usually referring to the "brilliance" of a stone
• Fire/Dispersion: The appearance of colored light return through the crown in conditions that enhance the metric
• Scintillation: Movement of the light's pattern within the diamond as the diamond, observer or light source is moved.
• Leakage: Areas of a diamond that do not return light towards the eye usually due to the light leaving the pavilion side of the diamond.

Diamond Brilliance

The arrangement of a diamond's facets (a key part of how a diamond is cut) determines the sparkle and brilliance of the stone. Facets of a diamond can be flat, angular, or surface plains. A diamond that is poorly-cut will lack sparkle and a diamond that is well-cut can compensate for a small size or imperfect color or clarity.

Depth and Width

The depth and width of the stone can affect the way light travels within the diamond and how it leaves the diamond, which directly impacts a diamond's brilliance. For example, if the diamond is too shallow, then light is lost through the bottom of the stone and causes a loss of brilliance. If it is too deep, then light escapes from the sides and creates a dark and dull appearance.

Brightness, Fire and Scintillation

The goal in extracting the most beauty from the diamond is to have the light enter the stone and disperse as it bounces inside the stone through the different facets and produces sparkle and flashes of brilliant color. While doing this, the diamond should return as much light to the eye as possible.

According to GIA:

A polished diamond's beauty lies in its complex relationship with light: how light strikes the surface, how much enters the diamond, and how, and in what form light returns to your eye. The result is a display of three attributes. Brightness is the combination of all white light reflecting from the surface and interior of a diamond. Fire is the colored flashes that can be seen in a diamond. Scintillation describes the sparkle of light you see in a diamond, and the overall pattern of bright and dark areas when you look at a diamond face-up.

A diamond is attractive when it is bright and fiery; when it has a pleasing, even pattern to its areas of light and dark (scintillation); and when it demonstrates a high level of craftsmanship. A diamond is less attractive if it is dark; if it is not fiery; if it has a distracting or unpleasant pattern; if it is carelessly polished; or if it is sloppily cut. Also, a diamond with a too-thin girdle may be attractive, but it is more susceptible to damage. A diamond with a thicker girdle or a larger total depth also can be attractive, but then the customer pays for unnecessary weight.

Anatomy of the Diamond

The cut of the diamond is determined by the anatomy of the diamond. The composition of the diamond is made up of diameter, table, crown, girdle, pavilion, culet, and depth.

• Diameter: Width of the stone that is measured through the girdle
• Table: Largest facet of a diamond
• Crown: Top part of the diamond that extends from the girdle to the table
• Girdle: Defines the perimeter of the diamond as it intersects the crown and pavilion
• Cut: Facet at the tip of the diamond.
• Depth: Height measured from the culet to the table

Diamond Refraction

Another process by which cut has a direct effect on how much light enters the diamond, and how brilliant the diamond will become, is a process called refraction. The brilliance of diamonds results from their high refractive index, a measure of how strongly a transparent material bends light rays. The key to a diamond's brilliance is that diamonds tend to bend light more so than any other natural substance.

Diamond Polish

Polish affects how light is able to pass through a diamond and is very important to a diamond's brilliance. You should only select a diamond that is laboratory certified with good, very good or excellent polish.

Diamond Symmetry

Symmetry is an important element of a diamond's finish. Symmetry refers to the size, alignment, and matching of the individual facets. You should only select a diamond that is laboratory certified with good, very good or excellent symmetry.