The Polariscope and its uses

The Polariscope 

The polariscope may be one of the most underestimated tools in gemology. Most gemologists use it to quickly determine if the stone at hand is isotropic or anisotropic or, at best, to determine the optic character of gemstones. With some small additions, one can determine both optic character and the optic sign of a gemstone. It is also the preferred tool -- next to the microscope -- for separating synthetic amethyst from its natural counterparts (although with recent synthetics that may prove difficult).
In addition, the polariscope may be very useful for distinguishing solid inclusions from negative inclusions as well as for spotting polysynthetic twinning.

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.