Fable  Gem Polariscope FTP-49 with  polarizing plastic sheets polarizing filters

Key Features:

To detect doubly or singly refractive stone. Able to tell if a DR stone has one optic axis or two. Able to detect strain in stones as diamond etc. Rotating glass polarizing filters on top and at the base. The opening in front can be used as light source for a refractometer.     

Basic

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:

• isotropic
• anisotropic
• 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 microcrystalline or cryptocrystalline aggregate (like, for instance, chalcedony).

4. The stone will show anomalous double refraction (ADR).

It is isotropic (single refractive).

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.