Refractometer Guide, Part 3: Determining Optic Character and Recognizing Optic Effects


Step 3: Practical Gemology

Lesson 6

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.

Optic Character

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.

Uniaxial

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 only one RI, you’ll have to test the stone again on another facet. The gem won’t rest on another facet as easily as it does on its table. You’ll have to hold the facet against the hemicylinder to do this. (Be very careful not to scratch your hemisphere! Remember …

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