The American Gem Society's Angular Spectrum Evaluation Tool visually provides much more information than can be garnered using a 'hearts and arrows' viewer or a Firescope
TM.
There is a fairly large range of weight retention and millimeter spread possibilities when setting the girdle at the halves and indexing the upper halves. This is really important when trying to make a 'size', i.e. 1.00 carat, from a given piece of rough.
When setting the girdle at the halves and indexing the upper halves to 1, you can improve the optical performance of the stone and its millimeter spread, but at the expense of weight retention.
When setting the girdle at the halves and indexing the upper halves to 3, you can improve weight retention, but at the expense of optical performance and millimeter spread.
The new AGS Grading System will lower the cut grade of standard round brilliants whose upper girdle facets cause optical performance to suffer.
Data
| | Girdle Thickness | Girdle Thickness | Weight @ 6.00 mm | Volume mm3 |
| | % at halves | % at mains | | |
| 1 index equal at halves | 3.7884% | 2.3583% | 0.774832586 | 43.999579 |
| 2 index equal at halves | 3.7884% | 3.7884% | 0.813705517 | 46.207014 |
| 3 index equal at halves | 3.7886% | 5.7261% | 0.866378294 | 49.198086 |
| | | | | |
| 1 index equal at mains | 5.2184% | 3.7884% | 0.817486295 | 46.421709 |
| 2 index equal at mains | 3.7884% | 3.7884% | 0.813705517 | 46.207014 |
| 3 index equal at mains | 1.8509% | 3.7884% | 0.808583102 | 45.916133 |