Someone recently asked me: “Do you use a polariscope to determine (locate) the C axis before you dop the stone?”
The short answer is “No, that’s not necessary”.
The longer answer is about why you don’t usually need a polariscope to locate C axis in materials where C orientation matters. There are three reasons to orient according to optic axis when dopping:
- Structural (as in managing cleavage, in a material like Topaz or Spodumene)
- Color (as in orienting for best color in a material like Corundum)
- Birefringence (as in minimizing the “fuzzy” effect that can come from facet doubling in a material like Zircon)
So, by the numbers (1), two of the most common structural concerns are the examples I used above – Topaz and Spodumene. Both of those materials usually have enough directional color that in a water-worn pebble (or even a polished sphere), a faceter who knows the material can orient the C axis readily just by knowing that color intensity is greatest on the C axis – and temp-tabling carefully.
When C axis orientation is a consideration for reasons of color (2), such as in the examples of Corundum, Spodumene, and Topaz – this is because the color is in fact different (better, more desirable, more intense) in that direction. So, anyone who can notice color can readily orient the rough to the best color direction. All that remains is to dop carefully in that direction.
Lastly (3), when facet doubling is a concern due to birefringence, one may use a polariscope to locate the optic axis. However, this is going to be a very rare case because facet doubling isn’t a standard consideration for an appraiser. It’s not going to reduce the value of the finished stone, while a reduced yield will. And, orienting strictly for optic axis in a gem where that doesn’t affect color may well degrade yield. In other words, unless there’s some unique and compelling reason that applies to a specific stone, there’s no justification for it.
A polariscope is a useful tool for many things, but a faceter with the standard knowledge we expect of Faceting Academy graduates won’t be reaching for one as part of the dopping process because they don’t need anything more than their eyes and their knowledge.
To learn more about opportunities for building this important and standard knowledge yourself, explore the Faceting Academy web site further.