Diamond cut refers to the proportions, polish, and symmetry of a stone: the three main factors involved in creating a diamond with optimal light reflection. Good light performance, the term used to describe light refraction and light return through the top of the diamond, is what many refer to as the "brilliance" of a diamond.
An excellent diamond cut produces a diamond with a high light return, dazzling brilliance, fire and scintillation. 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 fact makes a diamond's cut the most significant factor to influence appearance. For this reason, Zoara suggests that you choose the diamond with the highest cut grade that falls within your budget.
Brilliance is the total amount of light reflected by a diamond. When light hits the diamond's surface, some light enters and some is reflected back. The most immediate light reflected back is returned by the crown's angles.
Scintillation refers to the flashes of light, or sparkles, which are produced when a diamond is tilted from side to side. The light that isn't immediately reflected back enters the diamond and reflects from the inside walls towards the centre of the diamond. This light, which bounces off the internal walls of the diamond, is the quality described as scintillation.
A diamond's fire refers to the dispersion of light into different coloured light. Once the light is bounced off the inside walls towards the centre of the diamond, it then shoots back through the top of the diamond. Because the light is slightly bent by the diamond, a colour spectrum is visible when light exits the top of the diamond.
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
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 target table percentage for a round diamond is considered to fall between 53 and 59%.
A diamond's facets must be symmetrical in order to maximise 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-center culets. Zoara recommends that you consider round brilliant diamonds with a symmetry grade no lower than Very Good, and fancy cut diamonds with a symmetry grade no lower than Good.
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.Zoara recommends that you consider diamonds with a polish grade no lower than Good.
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. Most high end diamond retailers do not carry Poor cut diamonds.