If you’ve made it through our 3-part refractometer guide for gemologists, test your knowledge of refractive index (RI) analysis with this mini-quiz. Click on the link at the end of the quiz to see the correct answers. Got some wrong? Check out the refractometer review section below.

1. Parallax is caused by

  1. Double refraction
  2. A facet not being flat
  3. Lenses that need cleaning
  4. All the above
  5. None of the above

2. If you use a red filter with your refractometer, you need to make an adjustment to your reading of

  1. -0.005
  2. +0.589
  3. Neither
  4. Both

3. If you’re testing a gem and measured a high RI of 1.612 and a low RI of 1.608, the stone’s birefringence is 0.004.

  1. True
  2. False

4. A moderate-sized spinel inclusion in a spinel host gem will have a significant effect on its value.

  1. True
  2. False

For the answers, click here.

Refractometer Review

Question 1

Parallax is an optic effect that looks like the cutoff line wavering between scale divisions. It’s caused by a non-flat facet. Parallax is a common occurrence in refractive index testing that can be confused with an indication of gemstone birefringence.

Question 2

A refractometer should be calibrated at…