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

Step 3: Practical Gemology

Lesson 6

optic character - biaxial topaz
Topaz is a biaxial + gem. “Pink Topaz Pendant,” submitted by Laura. © Dan Stair Custom Gemstones. Used with permission.

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 …

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