Round Brilliant Cut Diamonds
A diamond's proportions and their relationship to each other primarily influence the GIA cut grade assigned. Grading the cut of a polished diamond begins with accessing brightness, fire, and scintillation. Every proportion of a round brilliant contributes to its interaction with light. If one proportion is "off" even slightly, a diamond could receive a lower cut grade. To understand cut grade it is important to know how proportions are evaluated and their relationships to one another.
Grading Proportions - Crown, Table, and Girdle
(Diamonds are always measured in millimeters)
Average Girdle Diameter
Determining average girdle diameter is the first step in evaluating proportions and is the starting point for comparing the diamond's other proportions. To determine average girdle diameter, measurements are taken from one edge of the girdle to the other across the diamond in several locations. Since no diamond is completely round it is necessary to take several measurements. After determining the measurements, add the smallest and largest numbers, divide by two, and round to the nearest hundredth (0.01) of a millimeter.
Minimum Girdle Diameter = 3.54mm
Maximum Girdle Diameter = 3.57mm
3.54mm + 3.57mm = 7.11
7.11 divided by 2 = 3.555
3.555 rounded to the nearest hundredth mm = 3.56mm
Total Depth Percentage
Total depth percentage determines why a diamond is underweight or overweight in relation to its diameter. After obtaining the diamond's average girdle diameter the total depth percentage can be calculated. To determine total depth percentage the diamond is measured from table to culet. After determining the measurement, divide the figure by the diamond's average girdle diameter, multiply by 100, and round to the nearest tenth of a percent.
Average Girdle Diameter = 6.52mm
Table to Culet Measurement = 4.02mm
4.02mm divided by 6.52mm = 0.61656
0.61656 x 100 = 61.656
61.656 rounded to the nearest tenth = 61.7%
A well-proportioned diamond will have a total depth percentage of approximately 60.0%. Diamonds with a total depth percentage below 55.0% are usually under weight. Diamonds with a total depth percentage of 65.0% or greater are usually over weight and can vary greatly in terms of reasons.
Note - although a diamond may have a total depth percentage of 60.0%, which indicates a well-proportioned diamond, it is important to note that the diamond may still have negatives such as a shallow crown, deep pavilion, or thick girdle. Also, total depth percentage ranges overlap between grades.
Table Facet and Table Percentage
The largest cut facet on a diamond is the table. The table facet along with the other facets allows light to enter and exit a diamond. The size of the table facet is an important factor when determining proportions. Table size is stated as table percentage, which is a percentage of the diamond's average girdle diameter.
Table size preference differs in markets throughout the world, but despite these differences a very small table (below 50%) or very large table (70% and greater) will have a negative impact on the diamond's face up (crown up-pavilion down) appearance, which will ultimately have a negative impact on the cut grade. Most diamond cutters today will cut diamonds with a table percentage of between 55%-65%.
Star Facet and Star Facet Length Percentage
Star facets extend from the edge of the table toward the girdle. They are very important to consider when evaluating a diamond's proportions in relation to the rest of the crown as they affect both brightness and fire.
Star facet length percentage is the length of the star facets expressed as a percentage of the total distance between the girdle and the edge of the table facet. Star facet length percentages usually range from 50%-55%. Beyond that range, very long star facets average 65%-70% and very small start facets range below 35%, which indicates the diamond is not symmetrical and will receive a lower cut grade.
The Crown - Crown Angle and Crown Height Percentage
Crown angle and crown height percentage are the two crown proportions that affect the diamond's appearance. Crown angle is the angle formed by the bezel facets and the girdle plane while the crown angle height percentage is the distance from the girdle plane to the table expressed as a percentage of the average girdle diameter. Crown angles between 25 degrees and 35 degrees produce bright diamonds with much fire. Crown height percentage is determined by the diamond's table percentage and crown angle.
The Girdle of a diamond basically prevents damage and provides an "edge" for setting the stone. A girdle can be bruted, polished, or faceted. Girdles should be thick enough to prevent chipping, but not too thick where it adds extra weight to the diamond. Thicker unpolished girdles can created a grayish reflection in the stone which can be unattractive and a girdle that is too thin can create durability problems as well as issues when setting.
Girdle thickness is usually reported in a range for example; thin-to-thick or thin-to-medium. Also, you may have heard of the terms "painting" and "digging out" both of which are weight saving techniques. Painting is a method to adjust the angles of facets to allow portions of the girdle to be thicker and digging out is a method leaving more girdle thickness to allow for clarity characteristics to be removed at a later date.