The pavilion of a round brilliant has two types of facets (not counting a culet if present): Eight pavilion mains and 16 lower girdle facets, also called "lower halves."
The pavilion main facets draw light from the highest angles. These are the engines driving light return. Between each of the 8 pavilion mains are 2 lower halves. In top performing
diamonds these lower halves have great influence on the life and character of the performance, particularly if the diamond has good patterning (optical symmetry or ‘Hearts & Arrows’).
What is Lower Half %?
The Lower Half % describes the average distance a diamond's lower girdle halves run on a straight line from girdle to culet.
Lower Half Examples
The distance the lower halves run from girdle to culet determine how thick, or narrow, the all-important pavilion mains are: Short lower halves result in wider pavilion mains. Long lower halves result in narrow pavilion mains. On most commercial round brilliant diamonds, the lower halves average between 75-85%, with the majority at the upper side of this range.
Estimating Lower Half Measurements
You can tell much about lower halves with reflector images made in such optical tools as a H&A viewer, ASET or Ideal-Scope. Here are crown views of diamonds with lower halves at 75, 80 and 85%. Note that these models assume perfect optical symmetry.
How do they differ?
In simplistic terms, short lower halves (wide pavilion mains) enhance diamond performance in indirect/soft lighting conditions. Long lower halves (narrow pavilion mains) enhance performance in very bright lighting conditions. All of this assumes well-performing proportions and good optical symmetry as a prerequisite, which is not always the case in commercial markets. Overall configuration has everything to do with the visual balance acquired. The very best diamonds perform equally well through a broad range of lighting conditions, thanks to carefully prescribed parameters.
AGS and GIA reports
AGS and GIA both state Lower Half % on their grading reports (GIA rounds the number to the nearest 5% however).
Our recommendations
Diamonds can have lower girdles cut to high numbers to enhance their attractiveness under bright jewelry store spotlights. However, taken away from those spotlights, such diamonds may not be as attractive in normal lighting. Precise configuration and optical symmetry must be taken into account, but in general terms our recommendations for best performance through a broad range of lighting conditions are for lower girdle lengths (GIA & AGS reports) 75-80%.
Hearts & Arrows Diamond Views
Reflector assessment is how Kazumi Okuda and his contemporaries developed optically symmetrical patterning decades ago. This approach is also the one Whiteflash used to fine-tune parameters for visual balance in Whiteflash A Cut Above Hearts & Arrows Diamonds.
Here are crown (arrows) views of diamonds cut to 78%, 80% and 82%. The differences are notable, though much less obvious than 75-80-85 seen above.
Below are diamond pavilion (hearts) views. This is the view most focused on for Whiteflash A Cut Above, as we feel the diamond's lower girdle-pavilion main relationships are of prime importance for light return and visual balance through a broad range of normal lighting conditions.
In this view even differences between 78%, 80% and 82% LGF are distinct.
78% - Robust hearts with slight separation from arrowheads above (pointing to the culet).
80% - Narrower hearts with notable separation from arrowheads. Slight splits appear at the Vs.
82% - Narrow hearts with significant separation from arrowheads. Notable splits at the Vs.
Summary
As splits become larger in the Vs of the hearts, some performance qualities are lost in lower lighting conditions. Different configurations behave differently but in general terms our recommendation for best performance through a broad range of lighting conditions are for lower girdle lengths (GIA & AGS reports) 75-80%.
Here is a dimmed pavilion view with crown facets wireframed in with stars at 50%.
Appendix for cut enthusiasts: Two ways to measure
Two approaches have been developed to measure a diamond's lower girdle facets. Lower Half Length % is the most common and describes the distance the lower girdle halves on a straight line from girdle to culet. Lower Half Height % describes the distance the lower girdle halves drop on an imaginary plumb line from girdle level to culet level.
AGS and GIA both report lower girdle length % on their reports, though GIA rounds the number to the nearest 5%.
Lower Girdle Height is expressed in laboratory research by AGSL and in DiamCalc software by OctoNus.
For comparison purposes, ‘height’ is 1-2% greater than ‘length’ in typical configurations. Example: 80% 'length' (on grading reports) = 81.5% 'height' (DiamCalc and AGS research).
For precise cross-reference, here are charts provided by the AGS, reproduced with permission.