The cut of a diamond refers to its proportions. Of the 4C's,the cut of a naturally mined diamond is the aspect most directly influenced by man while the other three factors are dictated by nature. Do not confuse the cut of a diamond with its shape. They are cut into various shapes depending on the original form of the uncut diamond (more commonly known as ‘rough diamond’). Regardless of its shape, a well-cut diamond is better able to reflect and refract light.
A diamond's ability to reflect light determines its display of fire and brilliance. Diamonds are usually cut with at least 58 facet or more. These facets follow a mathematical formula and are placed at precise angles in relation to each other. This relationship is designed to maximize the amount of light reflected through the diamond and to increase its beauty.
Well Cut
When a diamond is cut to proper proportions, light is reflected from one facet to another and then dispersed through the top of the stone. Within the Well Cut standards are the Diamonds.com sub categories of Ideal, Excellent, and Very Good.
Deep Cut
When the cut of a diamond is too deep, some light escapes through the opposite side of the pavilion.
Shallow Cut
When the cut of a diamond is too shallow, light escapes through the pavilion before it can be reflected.

The cut, or proportion, of a diamond is measured in percentages relative to the diameter of its girdle. The girdle diameter of each diamond is always considered 100%. Example: The girdle of a diamond measures 10 millimeters (100%). The table measures 5.6 millimeters. The total depth measurement is 6.1 millimeters. The diamond would be described as having a table of 56% and a depth of 61%. The table and depth percentages are the key to determining good proportions.
This range is very strict and combines the best in brilliance and fire. Technically, the crème de la crème.
This range is also of great beauty yet slightly more flexible regarding percentages. Many experts prefer the appearance of this range to Ideal.
Very Good
This range is balanced between precise proportions and price considerations. Viewed by many as the best overall value in beauty and price.
How Do I Know If A Diamond Is Well Cut?
A well cut diamond is the secret to a beautiful and brilliant diamond. Like beauty itself, the true meaning of “well cut” is often found in the eye of the beholder. While you may prefer a particular set of proportions, someone else might prefer slightly different proportions. Personal preference, even among experts, will always be an issue in defining the best cut. Think of Ideal, Excellent and Very Good as rings in a bull's eye. These classifications for cut represent an acceptable range for that category. The ranges narrow as you move toward Ideal at the center. Ideal has the narrowest range, with Excellent slightly larger and Very Good the largest. All three of these categories fall within the 'well cut' classification.

In many cases the visual differences from one classification to the next are so small they may be indiscernible to the naked eye. Science has produced sophisticated machinery that can measure every aspect of a diamond’s proportions. It is this precision that allows such strict standards to be defined. These guidelines give you the ability to not only make a selection regarding cut but to understand the underlying science upon which it is based. In any case a brilliant and scintillating diamond is ultimately the goal.

The Lowest score becomes the overall cut grade. In order to classify each diamond with an overall cut grade the lowest assigned grade for any individual characteristic is always used. For example: If the table percentage falls within Ideal yet the depth percentage is in the Very Good range, the diamond is classified as Very Good.

The proportions are not the same for every shape. Many of the diamond shapes require their own guidelines in order to achieve maximum beauty. Due to the mathematical differences inherent in different shapes, the table and depth guidelines are formulated to maximize fire and brilliance. Although the numbers may be different, the goal is the same: a magnificent display that highlights the individuality and character of each shape.
The girdle is the outer edge of a diamond. It usually has a frosted appearance. Many diamonds are also finished with a fully polished or even a faceted girdle. This characteristic does not affect the value of a diamond and is often more a reflection the diamond cutter's preference. The girdle is rated in terms of thickness. Girdle size is generally defined as either Extremely Thin, Very Thin, Thin, Medium, Slightly Thick, Thick, Very Thick, or Extremely Thick. The girdle can also be described as a range of these terms such as Thin to Thick. Avoid the two Extremes. When purchasing a diamond, select one with a girdle that is neither Extremely Thin nor Extremely Thick.
The culet is the bottom point of the diamond. In many cases this point actually has a very small facet. The culet is referred to in terms that relate to the presence or size of this facet. The culet is generally graded as None or Pointed, Very Small, Small, Medium, Slightly Large, Large, Very Large, and Extremely Large. Smaller is more desirable.
This characteristic refers to the finishing or final polishing of the facets, or flat surfaces. Contrary to common belief, diamonds are ground and polished, not chipped away, until they reach their final form. Each facet should be carefully fashioned by the diamond cutter to shine and be free from polishing imperfections. The polish of a diamond is generally defined as either Poor, Fair, Good, Very Good, or Excellent. When purchasing a diamond, select one with a polishing grade of Good or above.
This characteristic refers to the alignment and positioning of the facets, or flat surfaces. Each facet should be carefully positioned by the diamond cutter in proper proportion and relationship to the other facets. The alignment of each facet should be sharp and precise; improperly joined facet junctions can make a diamond appear uneven. The symmetry of a diamond is generally defined as either Poor, Fair, Good, Very Good, or Excellent. When purchasing a diamond, select one with a symmetry grade of Good or above.
This characteristic refers to the diamond’s ability to fluoresce under ultraviolet light. When exposed to UV light, many diamonds will give off a distinctive glowing blue coloration. Although fluorescence may be displayed in various shades, blue is the most common in diamonds. The fluorescence of a diamond is defined by its intensity as either None, Faint, Medium, Strong, or Very Strong. Most of the time fluorescence isn't an issue unless the intensity is Strong or Very Strong. In the very high colors D, E, and F, Strong fluorescence is considered less desirable. Ironically, in slightly lower colors of J and below, Strong fluorescence is desirable.
The best color for a diamond is no color. Diamonds allow light to be reflected and dispersed as a rainbow of color. This light dispersion, or color flash, has no effect on the technical grading of color. The absolute finest colorless stone carries a D rating, descending through each letter of the alphabet to Z, designating a diamond of light yellow, brown, or gray. This body color may be caused by the presence of trace elements, such as nitrogen, within the atomic framework of the carbon crystal. These trace elements are so minute that they are scientifically measured in parts per million (ppm). As the body color becomes more intense, the grade for color descends the scale. These gradations are so minute and precise that discerning a single grade (even by an expert) under less than ideal laboratory conditions is extremely difficult. It is always best to compare diamonds graded by either the Gemological Institute of America (GIA) or the American Gem Society (AGS) for accurate color grading. When directly comparing diamonds for color, most consumers are unable to detect a difference unless they are at least two or three color grades apart.
Fancy Color
It is often surprising to learn that diamonds also occur by rare accidents of nature in shades of pink, blue, green, amber, or even red. These rarely occurring colors are referred to as fancies and are evaluated by a different set of color standards. These standards take into consideration various factors such as hue and saturation. Fancy colored diamonds are the most expensive because of their extreme rarity. Some fancy colors can cost hundreds of thousands of dollars for diamonds of one carat or less!
Fluorescence is not directly related to a diamond’s color. This separate characteristic refers to the diamond’s ability to fluoresce under ultraviolet (UV) light. Our sun emits some UV light, but it is usually not great enough to detect fluorescence. The most common source of UV is a black light. When exposed to UV light, many diamonds will give off a distinctive glowing blue coloration. Although fluorescence may be displayed in various colors, blue is the most common in diamonds. The fluorescence of a diamond is defined by its intensity as either None, Faint, Medium, Strong, or Very Strong. Although fluorescence is a characteristic that can be measured, it is seldom an issue when selecting a diamond.
Almost all diamonds contain very tiny natural birthmarks known as inclusions. To determine a diamond's clarity, an expert views it under 10 power magnification. In addition to internal inclusions, surface irregularities are referred to as blemishes. These two categories of imperfections-inclusions (internal) and blemishes (external)-make up clarity. The fewer the imperfections, the rarer and more valuable the diamond. Many inclusions are not discernable to the naked eye and require magnification to become apparent. A laboratory-certified clarity rating of SI2 represents the point at which inclusions are technically not apparent to the average naked eye.

Contrary to popular belief, higher clarity does not always mean more beautiful. If the inclusions are not visible to the naked eye, a higher clarity does not really improve the appearance of a diamond but rather the rarity and price. A higher clarity is more desirable and valuable, but knowing that you have selected the right clarity for the right reasons is most important. We recommend a clarity of SI2 or better.

Clarity is graded using a very precise and complex method of evaluating the size, location, and visibility of inclusions. The diagrams to the right show a top view of a round diamond. The inclusions, shown in red, are an approximate sample for each clarity rating. The plotted inclusions may not be actual size for display purposes. Remember, the inclusions depicted in red are not visible to the average naked eye until the I1-I3 clarities. Below is the technical clarity scale with a description of each term.
Most people compare carat weight to size. The larger the diamond the more it weighs. The weight of a diamond is expressed in carats. The word carat originated from the carob tree or Ceratonia siliqua. The tiny seeds of this tree are well known for their uniformity and consistent weight. Traditionally diamonds and gemstones were weighed against these seeds until the system was standardized, and one carat was fixed at 0.2 grams. One carat is divided into 100 points. A diamond weighing one quarter of a carat can also be described as weighing 25 points or 0.25 carats. Points are generally not used to describe weights over one carat. Here are some examples of different weights for round diamonds and their corresponding sizes. These may not be actual size due to your monitor. The approximate girdle diameter is displayed in millimeters.
How Size Effects Rarity
The rarity of a diamond is greatly affected by its size. The rarity of a 1.00 carat diamond is much greater than twice that of a .50 carat. Although it only weighs twice as much, the 1.00 carat is statistically much more difficult (rare) to mine than the .50 carat. For an easy comparison of price and size. Prices are approximate and based upon D Color, internally flawless, excellent cut.
Diamonds are cut in many different and exciting shapes. The shape of a diamond is often confused with its cut. Shape refers to the basic form of the diamond: oval or pear shaped, for instance. Cut or proportions, on the other hand, refer to the ability of each of these shapes to reflect light. A round diamond, for example, could have a good cut or a poor cut depending upon its proportions. When it comes to shape, it is simply a matter of personal taste. The right shape for you is really the one whose appearance you prefer. Shape can be a statement of whom you are; like other areas of fashion, shape can reflect your individuality. The most popular shapes are displayed here, but many new and interesting shapes are being developed every year.