Step 3: Practical Gemology

Lesson 6

Before learning to determine optic character, be sure to review our article on using a refractometer to determine the RIs of faceted gems. You should be familiar with the instrument, procedures, and testing terminology.

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 hemicylinder! Remember to shield it from extraneous light with your free hand, too).

If…