A large culet on a diamond allows a surprisingly small amount of light loss because most ray paths from light entering the crown of a diamond travel at angles that will be totally internally reflected. A visible culet mars the appearance and often appears as a black inclusion.
If a culet size is smaller than one percent no penalty is added. Above one percent on a sliding scale a penalty added. At five percent culet size the penalty is 0.8.
A better system would be a measurement based culet penalty, but the complexity of data entry makes this too difficult until we finalise direct input software from Sarin reports.
| Table 9. Culet adjustment | ||||||
| Culet size | 1% | 2% | 3% | 4% | 5% | 6% -> |
| Score | 0 | 0 | 0 | 0.2 | 0.4 | 0.6 ->< /font> |
There is also a written warning for larger culet sizes.
It is worth noting here that the trade commonly refers to angles when reporting diamond crowns, but uses depth percentages in reference to pavilions, possibly a hangover from pre-scanner times when pavilion depth was estimated (erroneously) from table reflections. It is unreliable to report depth percentages because pavilion angles increase as culet size increases. Not only is pavilion angle the most critical proportion on a diamond, but because it is the longest facet it is also the most accurate scanned proportion. (OctoNus Helium scanners have a clear advantage over their competitors products in this respect.) Calculating pavilion angle from depth percentage and culet size requires charts or a mathematical mind. Responsible authorities should no longer list depth percentages.