The polariscope (polarization)

 Basic

A polariscope is a gem identification tool that employs polarised light. There are two polarised filters on the instrument, one on top and one on the bottom. Both the polarizer and the analyzer have vibrational planes of their own. The field between them stays black when the polarizer's vibrational plane is at right angles to the analyzer's vibrational direction. The "crossed position" is the name for this pose. Gems can be examined in this position to see if they are: isotropic anomalously double refractive (ADR) anisotropic anisotropic aggregate

Operation of the polariscope and possible observations

Turn on the light source and set the gemstone on the rotating platform immediately above the polarizer with the polarizer and analyzer crossed (this platform might not always be present, in which case you use your tweezers). You'll get four options if you look at the gemstone via the analyser while gently spinning it. 1. The stone appears dark throughout a 360° rotation. The stone is isotropic (singly refractive). 2. The stone will show anomalous double refraction (ADR). It is isotropic (singly refractive). 3. Throughout a 360° rotation the stone blinks 4 times, light and dark. The stone is anisotropic (double refractive). 4. The stone will appear light all the time. The stone is a microcrystalline or cryptocrystalline aggregate (like, for instance, chalcedony).

Further reading

Standard Gemological Instruments and their use