The beauty of a diamond depends more on cut than any other factor. While all the other Cs are largely determined by Nature, cut is the one that can be determined by Man. For this reason, cut is perhaps the most important of the four Cs for us to understand.
Cut refers to the proportions, polish, and symmetry of a stone: these three factors are important in creating a diamond with the best possible light reflection. The angles and finish of any diamond are what determine its ability to handle light, and its brilliance.
When a diamond is well-cut, light enters through the table and travels to the pavilion, where it reflects from one side to the other before reflecting back out of the diamond through the table and to the observer's eye. This light is the brilliance, and it's this sparkling blaze that makes diamonds so captivating.
A poor cut, on the other hand, can cause light to seep out of the sides and bottom of the diamond, or it can limit the amount of light that enters a diamond. Poor cuts can therefore cause the diamond to appear dark, dull and lifeless, despite its colour and clarity grades. This is why cut is the most significant factor in the appearance of a diamond.
Brilliance, Fire and Scintillation
The cut of a diamond has three primary effects on appearance: Brilliance, Scintillation, and Fire.
Brilliance refers to the brightness of a diamond, created by the combination of all the white light reflections from the surface and the inside of a polished diamond. When light hits a diamond's surface, some light enters and some is reflected back. The amount of light immediately reflected back depends upon the crown's angles.
The cut of a diamond also creates “contrast”, the light and dark areas seen in a diamond. These dark areas are not inclusions, but shadows created by the cut or by objects between the light source and the diamond (such as the observer's head).
The greater the amount of reflected light and the stronger the contrasts, the more brilliant the stone. A round diamond will typically exhibit more brilliance than a fancy shape due to the superior mechanics of the round cut for reflecting light.
Scintillation refers to the flashes of light, or sparkles, which are produced when a diamond is tilted from side to side. These flashes are caused when light that isn't immediately reflected back enters the diamond and bounces off the internal walls. They are most pronounced in flood lit areas where strong light enters the diamond from multiple angles.
The light that reflects out of a diamond can appear in brilliant white flashes, or in a rainbow of colour, referred to as Fire. The fire effect is caused by the proper bending and dispersion of light as it travels through the diamond. The more colourless the diamond, the truer the dispersed colours will appear. This effect is greater in darker areas where there are fewer light sources.
Maximum brilliance is achieved in diamonds that posses both greatest fire and best scintillation.
Understanding Diamond Anatomy
To properly understand a diamond's cut, it is important to understand the terminology of basic diamond structure as it relates to proportion, symmetry and polish.
Diameter: The diameter is the width of a polished diamond from one side of the girdle to another
Table: The table is the largest polished facet of the diamond on the top face of the stone
Crown: The crown is the top part of the diamond that is measured from the surface of the table to the girdle
Girdle: The girdle is the widest edge of the diamond where the crown ends and meets the pavilion
Pavilion: The pavilion is the bottom part of the diamond that begins at the girdle and extends downward to the point of the culet
Culet: The culet is the tiny flat facet at the bottom tip of the diamond
Depth: The depth of a diamond refers to the total length of a diamond, measured from the culet to the table
Proportion, Symmetry and Polish
Diamond proportion refers to the relationship between the size, shape, and angle of each facet of a diamond. A wide range of combinations are possible, ultimately determining the diamond's interaction with light.
When light strikes a diamond, approximately 20% immediately reflects off the surface (as glare). Of the 80% that enters, a portion will escape through the bottom of the diamond (where the observer cannot appreciate it). A well proportioned diamond will have each facet properly placed and angled so as to maximize the amount of light that reflects back out of the crown (top) of the diamond, to the eye of the observer.
To optimally capture light and reflect it back, a diamond's pavilion must have accurate angles and depth. If the angle of the pavilion is too shallow or too deep, light will escape or leak out, creating dark and dull "stains". The crown angle is also extremely important since this affects the way that light enters and exits the diamond.
Not only are the angles important, but depth percentage and table percentage are also key factors that contribute to the quality of a diamond's cut. Depth percentage refers to the depth of the diamond divided by its diameter. Shallower diamonds have low depth percentages whereas deeper diamonds have higher depth percentages. A good target depth percentage for a round diamond is considered to fall between 59 and 62.5%. Table percentage refers to the width of the table divided by the diameter. Again, diamonds with a higher table percentage have larger tables, and diamonds with a smaller table percentage have smaller tables. A good table depth percentage for a round diamond is considered to fall between 53 and 59%.
A diamond's facets must be symmetrical in order to maximize the amount of light that enters and exits the stone. Diamonds with poor symmetry look slightly distorted, unbalanced and improperly shaped. Moreover, they will affect brilliance, scintillation and fire. Many asymmetrical round stones are not completely round, or have misshapen facets or off-centre culets.
Once a diamond is cut, each facet of the diamond is polished. If the polishing is done improperly, it can leave scratches and streaks that are similar to the marks left behind after a car waxing. An Excellent diamond polish is a diamond which has very few or no scratches.
Ideal/ Excellent – Excellent light performance. Reflects almost all of the light that enters. Rare and extremely beautiful cuts.
Very Good – Very good light performance. Reflects almost all of the light that enters. Very Good diamond cuts are considered to be an outstanding value.
Good – Good light performance. Reflects most of the light that enters. Good diamond cuts are far less pricey than Very Good cuts.
Fair – Not as brilliant as a Good cuts or above, Fair diamond cuts are still considered good quality diamonds.
Poor – Poor cut diamonds are typically cut too shallow or too deep causing much of the light to leak out of the diamond's sides and base.
Which Grade of Cut Should I Buy?
Cut grade is the most important factor in determining the overall appearance of a diamond, because a poorly cut diamond will seem dull even with excellent clarity and colour. Conversely, a well cut diamond can have a slightly lower colour (G-H) or clarity (SI1-SI2) and still look quite beautiful, due to its superior ability to create sparkle and brilliance.
For superior brilliance, choose a diamond with a Cut grade of Very Good or Excellent for round diamonds, and Good or better in fancy shape diamonds. When choosing a diamond in this range, make sure its Symmetry and Polish are Very Good or Excellent, so that the impact of the above average Cut is not obscured.
For those on a budget, primarily concerned with size, a diamond of Fair - Good cut may be an acceptable choice, especially in fancy shapes. While the diamond will lack the scintillation and brilliance of a well cut diamond, it will allow a significant increase in size for the same price.
Avoid Poor cut diamonds, even if size is the primary concern. Most find these diamonds to be an unacceptable trade off, despite the lower price.
The number one mistake made when purchasing a diamond is to be misled on cut quality. Cut is more difficult to define than color or clarity, and therefore often ignored or misrepresented. Common issues include:
Being shown two or three diamonds of various cut qualities, in an effort to sell the best of the available options. While the customer may choose the best option shown, it is not necessarily a well cut diamond. It is simply the best of what is currently available at that particular store.
Purchasing a deeply cut diamond. A deeply cut diamond carries more of its carat weight "hidden" in the depth of the diamond as opposed to the width. These poorly cut diamonds are less expensive per carat, and are common in most jewelry stores. A customer might purchase a 1.00 carat diamond that actually looks like a .90 carat diamond because it is too deeply cut.
Because well cut diamonds are more expensive per carat than fair or good cuts, few are carried in traditional jewelry stores. Less than well cut diamonds cost less to purchase, less to inventory, can be sold at a lower price, and turn more quickly in the jeweler's inventory - so the incentive to carry them is overwhelming.
A diamond’s "cut" is different from its "shape." “Shape” refers to the general outward appearance of the diamond, (such as round, emerald, or pear). "Cut” is a reference to its reflective qualities.
If you opt to buy a diamond without a certificate, spend some time looking at certified diamonds (where you know the Cut Grade) and train your eyes to identify the better cuts (by their "sparkle").
Remember that observable fire is increased in a slightly darker room (such as a candle light). Conversely, scintillation is maximized in strongly lit areas (such as an office). Fire and scintillation are simultaneously maximized by very strong, pinpointed light sources (such as the spot lights prevalent in jewellery showrooms).
Unlike the other "Cs" (carat weight, colour, and clarity), the various Cut grades in existence today were not originated by GIA, and are not uniformly applied. In fact, GIA has only been assigning cut grades since 2005, and only to round diamonds. Even though retailers use common terms to describe Cut (such as Excellent, Very Good, Fair, Poor) the terms are not uniformly defined or applied. In fact, a diamond seller may assign any cut grade they choose, based on any set of factors they wish.