Lesson 6: Refractometer Guide, Part 3: Determining Optic Character and Recognizing Optic Effects
Step 3: Practical Gemology
Optic character is another useful clue in gem identification. This property exists in every doubly refractive gem but not in singly refractive or amorphous materials. The optic character of many gems can be determined with a polariscope, but you’ll need a refractometer to test some gems.
There are five possible optic characters:
- Uniaxial positive
- Uniaxial negative
- Biaxial positive
- Biaxial negative
- Without sign
You can usually ascertain the optic character of a gem from the notes you took while testing it for double refraction. Interpret the results as follows.
The gem is uniaxial if both refractive indices (RIs) remain the same in each position or if one RI remained the same and the other varied. (While doing this, you should have found a position with only one RI).
If you didn’t find a place where there was …
Every Gemstone Is Unique - Here's How The Experts Evaluate Them
in this training-course, discover the tools, tests & valuation process used by expert gemologists for colored gemstones, opals & diamonds