What is a polariscope used for in gemology?

• A. To measure refractive index numerically.
• B. To observe absorption lines in spectrum.
• C. To detect birefringence; isotropic stones appear dark, anisotropic stones show interference colors.
• D. To test thermal conductivity.

Answer: (C)

Birefringence is what a polariscope reveals. In crystals with anisotropy, light splits into two rays with different speeds as it passes through, creating interference colors when viewed with polarized light. A polariscope uses crossed polarizers so light from an isotropic stone is extinguished, appearing dark, because there is no double refraction to produce colors. In contrast, an anisotropic gem shows interference colors in the field or through the analyzer due to the two refracted rays traveling at different speeds. This makes the polariscope a quick way to tell if a gem is isotropic or anisotropic, which helps with identification and distinguishing certain gems. This function is different from what a refractometer does (measure refractive index numerically), a spectroscope (observe absorption lines), or a thermal conductivity tester (test heat transfer).