A polariscope uses polarized light for gem identification. It consists of two polarized filters, one on the top and one on the bottom of the instrument as seen in the picture to the right. Both the polarizer and the analyzer have their own vibrational planes. When the vibrational plane of the polarizer is at right angles to the vibrational direction of the analyzer, the field between them remains dark. This position is known as the "crossed position". In this position, gems can be tested to determine if they are:
- anomalously double refractive or an
- anisotropic aggregate
The polarizing filters of this instrument are made of polarizing plastic sheets (polyvinyl alcohol containing dichroic molecules stretched-polymers). Older models were created with microscopically oriented crystals of iodoquinine sulfate (herapathite) or tourmaline plates.
Operation of the polariscope and possible observations
With the polarizer and analyzer in crossed position, turn on the light source and place the gemstone on the rotating platform just above the polarizer (this platform might not always be present, in which case you use your tweezers).
Observing the gemstone through the analyzer while slowly turning the stone will give you 4 possibilities.
1. The stone appears dark throughout a 360° rotation.
- The stone is isotropic (single refractive).
2. Throughout a 360° rotation the stone blinks 4 times, light and dark.
- The stone is anisotropic (double refractive).
3. The stone will appear light all the time.
- The stone is a micro crystalline or crystalline aggregate (like for instance, chalcedony).
4. The stone will show anomalous double refraction (ADR).
- It is isotropic (single refractive).
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