In the course of GIA’s research into the effect of proportions on the appearance of a round brilliant cut diamond, we have looked at many aspects of this cutting style, including a number of diamonds with cuts that have interesting variations from what is considered standard. The round brilliant is typically described by its proportions and angles: total depth percentage, crown height percentage, pavilion depth percentage, table percentage, average crown angle, and average pavilion angle (along with girdle size and condition, culet size, and finish quality). In our article in the Fall 2001 issue of Gems & Gemology, we pointed out the significance of two other parameters – the lengths of the lower girdle and star facets (the latter determines the length of the upper girdle facets as well). These parameters define the positions and areas of the star, upper girdle, and lower girdle facets, which typically cover more than half the surface area of a round brilliant. The importance of these facets, which was noted briefly by Marcel Tolkowsky in his 1919 treatise on cut, is again being recognized after decades of omission from proportion-based diamond cut grading systems.

However, the above description does not include another important aspect of a diamond’s shape: the orientation of the upper and lower girdle facets (sometimes referred to as “half” facets). In a standard round brilliant diamond, these facets are evenly spaced around the diamond. Yet upper and lower girdle facets can also be polished so that they lean toward the bezel or pavilion main facets and away from each other (facets created in this manner are referred to by diamond cutters as “painted” facets). This change in orientation reduces the angle to the horizontal of the upper and lower girdle facets, and yields a shallower angle between the half facets and the adjacent bezels and pavilion mains. Alternatively, the half facets can be fashioned so that they lean toward each other, thereby creating steeper angles between each upper or lower girdle facet and its neighboring bezel or pavilion main facet, and steeper angles of the halves themselves with respect to the horizontal (in which case they are referred to as “dug out” facets). These techniques are well known in the diamond cutting industry, but seldom discussed outside of that group.

Another way to describe this effect is with a term primarily used by colored stone cutters, namely “indexing.” In the design of a given colored stone cut, the position of the center of each facet around the outline of the stone is described in reference to the index wheel used in cutting machines (see, e.g., G. and M. Vargas, Faceting for Amateurs, 3rd ed., publ. by Glenn and Martha Vargas, Thermal, California, 1989). Such wheels come in a variety of scales, dividing the circular outline into, for instance, 64, 80, or 96 steps. “Indexing” is the practice of altering the “standard” positions for some facets – that is, moving their centers slightly in either direction, so that their three-dimensional position is changed. Index wheels usually are not used in diamond cutting because of hardness constraints on the positions and directions of cutting and polishing, but the indexing technique (“painting” or “digging out”) is still practiced (figure A).

Normally, the centers of half facets occur every 22.5° around the girdle. Full polishing of the facets in these positions gives the girdle its evenly scalloped shape (figure B, center). Without changing the lengths of the upper or lower girdle facets, their index positions can be altered by a few degrees, either toward the bezel or pavilion main (figure B, left), or away from them (figure B, right). These changes in position affect the scalloping of the girdle, because they change the area and shape of the half facets, and their angle to the horizontal. If the girdle were a circle with no thickness, crown and pavilion half facets with altered index positions would overlap, and the odd facet shapes that resulted would be glaringly obvious. Instead, the differences in facet shape are subtle, and are only visible from the profile view.

Such changes in the upper  and lower girdle facets also affect the weight recovery of the stone. As shown in figure B (left), leaning these facets toward the bezel or pavilion main facets – “painting” the facets – results in a thicker girdle scalloping at the junction between the halves, which yields a greater weight in the finished diamond for the same total depth. “Digging out,” or leaning the half-facet positions the other way (toward the junction between the “halves”; figure B, right), causes the scalloping to change in the other direction (the thickness of the girdle scalloping at the junction between the halves is smaller than the bezel/main girdle thickness) and yields a lower finished weight.

Changing these index positions also changes the angle between the two half facets, and the angle between each “half” and the bezel or pavilion main next to it. Such variations in the inter-facet angles significantly affect the movement of light through the diamond, and thus its overall, face-up appearance. Figure C shows three round brilliants with similar proportions (table 1). These diamonds were photographed in a viewing environment that uses both diffuse white light and a black area that emphasizes the face-up contrast pattern of a diamond (areas of contrast may look different in actual face-up views of diamonds in normal lighting and viewing conditions). Despite the similar proportions of the three diamonds, they display markedly different face-up patterns. These differences are mainly due to the placement of the upper and lower girdle facets.

The round brilliant on the left is a commercial diamond in which the upper and lower girdle facets have been displaced toward the bezels and pavilion mains (as in figure B, left). This diamond shows little contrast except for the dark appearance of the pavilion mains beneath the table. The diamond in the center, from the Hearts On Fire brand, has all its facets – including the “halves” – in evenly spaced positions (as in figure B, center). This diamond shows areas of contrast both under the table and under the eight bezels. The round brilliant on the right, called the Fabrikant Brilliant, has half facets that have been moved toward each other (as in figure B, right), which increases the number of areas of contrast seen under the table and adds eight semicircular areas of contrast around the edge of the diamond. In this case there is a noticeable difference in the location of areas of contrast, which changes its appearance.

The three diamonds in figure C are examples of different diamond appearances that can be achieved within this combination of proportions. Still, other distinct appearances can be created from these proportions by other variations in the placement of the upper- or lower-girdle facets. Truly, every facet matters.

Ilene Reinitz and Tom Moses
GIA Gem Laboratory, New York