What is a round brilliant cut?

A round brilliant cut diamond is one that has 57 facets, or 58 facets including a culet . This cut is also known as the American ideal cut , old European cut, American standard cut, modern brilliant cut and brilliant cut. With varying standards of reference, this cut is called ideal as it works towards the overall purpose of giving a diamond with maximum fire and scintillation. While the round brilliant cut take longer cutting time than other cuts and lead to more loss of carat weight than standard cuts (an 8mm diameter ideal cut diamond weighs approximately 1.91 carats), these stones are renowned for their ideal crown, pavilion and girdle symmetries. It is also known that round brilliant diamonds are usually cut from octahedron crystals and from small crystals, with the loose stone resembling the shape of a cone.

History of round brilliant cut

This cut is said to have emerged from the old miner cut or the triple cut which was popular in the seventeenth century. While these ancient cuts had all facets similar to the current day round brilliant cut, the facet alignments were noted to be different. This cut gave direct rise to the Old European cut on which many experiments were done to define the perfectness of the current round brilliant. Soon Marcel Tolkowsky, a renowned Belgian diamond cutter, took up empirical calculations as a part of his PhD thesis in mathematics and gave the diamond industry the renowned Tolkowsky cut in 1919.

Types of round brilliant cut

While the number of facets for round brilliant diamonds is fixed, the proportions are not. This is what gives range to a wide variety even in round brilliant diamonds. Currently there are six different ideal cuts devised over the years. These include the,

American ideal cut

This is also known as the modern round brilliant cut or the Tolkowsky cut. This cut is the benchmark of ideal cuts in North America. As per Tolkowsky who, the ideal cut must have a depth percentage of 59%, table percentage of 53%, crown height of 16%, pavilion angle of 40.75 degrees, crown angle of 34.5 degrees and pavilion depth at 43.1%. While the Tolkowsky ideal does not specify a girdle, in reality a round brilliant must have a medium and even girdle thickness to prevent chipping while the diamond is set. Besides this perfect symmetry, a minute or absent culet and perfectly aligned facets are pertinent. 

Eppler cut

One variation of the modern round brilliant cut is the Eppler (European Practical Fine cut) which is the standard in Germany and other European countries. It was discovered in 1939 and is slightly different from the American ideal cut and is noted to increase the table size by 3%. This cut has a table size of 56%, crown height of 14.4%, overall height of 57.7%. The reduction in crown height leads to maintaining dispersion similar to the American ideal. 

ScanDN

Scan DN (Scandinavian Diamond Nomenclature) cut is noted to have table width of 57.5%, overall height of 57.7% and crown height of 14.6%.

Other cuts

Ideal brilliant, Parker Brilliant and Eulitz Brilliant are the other types of ideal cuts. Among these, the Parker brilliant and Eulitz brilliant have been disused as the proportions cause low brilliance.

Dimensions of the cut

This cut has 58 facets, which includes the culet. In cases where the culet is not present, round brilliants are noted to have 57 facets. The crown or the upper portion of the diamond comprises of one table facet, 8 bezel facets, 8 star facets and 16 upper girdle facets. Besides these 33 facets, the pavilion or the bottom portion of the diamond comprises of 16 lower girdle facets, 8 pavilion main facets and one culet. The dimensions of factors like table size, crown angles and pavilion depth are taken into consideration while measuring the brilliance of a diamond. However as per GIA and many other laboratories, it is considered that other factors like total depth, girdle thickness, culet size, star facet length, lower girdle facet length, crown height and finish are taken into consideration while measuring the brilliance of the diamond. GIA has demonstrated that a slight variation in one factor can alter the brilliance of the diamond. Among the above dimensions, GIA reports that the relationship between brilliance of a diamond and the primary proportion factors of crown angle, pavilion angle and table size is rather complex. Also a well cut round brilliant needs to have good balance between fire and brilliance, with the stone reflecting light upwards and making the diamond appear white. While variations in any of the above factors affect the brilliance of the diamond in different ways, these three factors specially produce unusual results. Here are the specifications.

Table size

GIA specifies a table size of 56% as ideal while AGS notes a range from 53% to 57%. It is noted that as the table size of a diamond changes, the difference in the brilliance of a diamond is significant, also affecting the crown height and total depth of the diamond.

Depth percentage

This refers to the total depth of the diamond divided by the overall diameter. Tolkowsky specifies the standard at 59.3% and it is known that that this factor is the quickest indicator of quality of cut for a round brilliant. While a slight increase of up to 62.5% is acceptable, more increase in depth percentage reduces the brilliance of a diamond.

Crown angle

The crown diffuses light into various colors, and is an important parameter for the ideal diamond. 34 to 35.5 degrees is considered ideal for round brilliant diamonds. While a change in the crown angle does not show any great difference in the face up view, the profile view is greatly altered, again affecting the crown height and total depth of the stone.

Star facet length

Set at 50%, a change in this parameter causes great difference in face up view than in profile view. Also as the crown height and crown angle are not affected, the change in star facet length is not considered a pertinent factor for measuring brilliance.

Girdle thickness

Given a quantitative 3% by GIA, more thickness in the girdle increases the total depth of the diamond. Tolkowsky specifies a no girdle, however in reality it is required to prevent chipping in a diamond. It is thus measured at 1% to 2% of total diameter. Further increase in girdle thickness adds weighted look to a diamond, when it is viewed with the face up.

Pavilion angle

The pavilion functions by reflecting back the light through the top of a diamond, and thus slight change in the pavilion angle brings difference in the profile view. However the pavilion up view is not affected by minute changes. Pavilion depth and total depth are affected by changes in the pavilion angle.

Culet size

A pointed culet or the absence of a culet is considered the best for ideal diamonds. A wider culet is seen to bring about a marked difference in how the pavilion appears. Both total depth and pavilion depth are affected by changes in this parameter.

Besides the above factors, the overall diameter of a diamond is also a quick indicator of its brilliance. A round brilliant diamond must have a diameter (in millimeters) equal to 6.5 times the cube root of its carat weight. For example, a diamond of 1 carat must have a diameter of 6.5mm.

Cut grade for round brilliant diamonds

Different laboratories grade diamonds differently. The differences of grading used by a few of the laboratories are specified as under, both in quantitative and qualitative form.

GIA

The GIA diamond lab ideals have fixed reference values. It bases its cut grade for round brilliant diamonds on its experience of fifteen years, sophisticated computer modeling technologies like ray tracing and thousands of observations. Its cut grades fall in the categories of excellent, very good, good, fair and poor. Approximately 99% of diamonds graded at GIA come in the top three categories. Elements of physical design such as weight, durability and craftsmanship are also taken into consideration. GIA also uses its trademarked GIA Facetware products like cut estimators to ensure that results are predictable. GIA round cut diamonds are also featured in our search resutls.

AGS

Unlike GIA, AGS provides a range within which values are acceptable. Grades here are based on its five years of research on the light performance in diamonds. The distance of the diamond from the eye is noted to be at 25cms. The laboratory uses their AGS Performance Grading Software, the first of its kind to be used for round brilliant diamonds. It grades all 58 facets of round brilliants and measures them in three dimensions (most other labs grades diamonds on two-dimensional basis). The three factors of polish, symmetry and proportions are taken into consideration while grading. Triple ideal grading is awarded when all three parameters are in harmony. Instead of stringently quantifying different facet angles and facet ratios, the laboratory provides grading depending on where different facets intersect. AGS round cut diamonds are also featured in our search results.

Other diamond laboratories

These include labs like AGA and HCA. Amongst these, the standards of AGA are considered overtly strict. HCA on the other hand, is said to have altered its standards in 2004, to distinguish between brilliant and Tolkowsky. The specifications are measured based on a distance of 40cms from the eyes.

Standard specifications by a few laboratories are given as under,

Different laboratories also use different kinds of sophisticated equipment to measure the cut quality. These include H&A viewers, BrilliantScope, Idealscope, FireTrace and SarinDimension.

While the ideal cut is very desirable, the definition of such a cut is quite subjective and often a matter of controversy amongst many diamond experts. With various theories existing on the ideal cut and its proper parameters, quality and technique of cut are vital factors. These not only make a diamond look bigger than it actually is, but also give it more brilliance. The basis on which customers decide the right ideal diamond is to judge how beautiful it looks. While the round brilliant cut is both desirable as it is expensive, beauty lies in many other cuts also. Judge each stone by its merits and not with technical jargon. This is what will make the purchase of a breathtaking diamond very easy.

Here is a collection of excellend round diamond cut featured in our search results.