Video showing the behavior of double refractive stones and ADR under the polariscopeĪ possible solution for overcoming the confusion when one suspects ADR is to orientate the stone in its lightest position and then quickly turn the analyzer 90°. Video illustrating the use of the polariscope to determine optic character The first 3 behaviors should pose no problems for the inexperienced user, but the latter (ADR) can be misinterpreted and cause one to think the stone is double refractive. The stone will show anomalous double refraction (ADR). The stone is a microcrystalline or cryptocrystalline aggregate (like, for instance, chalcedony).Ĥ. The stone will appear light all the time. The stone is anisotropic (double refractive).ģ. Throughout a 360° rotation the stone blinks 4 times, light and dark. The stone is isotropic (single refractive).Ģ. The stone appears dark throughout a 360° rotation. Observing the gemstone through the analyzer while slowly turning the stone will give you 4 possibilities.ġ. 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).
Operation of the polariscope and possible observations Older models were created with microscopically oriented crystals of iodoquinine sulfate (herapathite) or tourmaline plates. The polarizing filters of this instrument are made of polarizing plastic sheets (polyvinyl alcohol containing dichroic molecules - stretched polymers). In this position, gems can be tested to determine if they are: This position is known as the "crossed position".
When the vibrational plane of the polarizer is at right angles to the vibrational direction of the analyzer, the field between them remains dark. Both the polarizer and the analyzer have their own vibrational planes. 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. A polariscope uses polarized light for gem identification.
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