Polariscope

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Revision as of 08:03, 23 September 2006 by Doos (talk | contribs) (Operation of the polariscope and possible observations)
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--Doos 05:36, 7 September 2006 (PDT)

The polariscope is maybe 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 optic character in gemstones, while with some small additions one can determine both optic character and optic sign of a gemstone. It is also the prefered tool (next to the microscope) to seperate synthetic Amethyst from its natural counterparts, although with recent synthetics that migh prove difficult.
Furthermore it may be very useful for distinguishing solid inclusions from negative inclusions aswell as spotting polysynthetic twinning.

Basic

Polariscope.jpg

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 above picture. Both the polariser and the analyser have their own vibrational planes. When the vibrational plane of the polarizer is at right angles to the vibrational direction of the analyser, 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
  • Anamolously Double Refractive or an
  • Anisotropic Aggregate

The polarising filters in this instrument are made of a plastic with microscopically oriented crystals of quinine idosulphate.


Operation of the polariscope and possible observations

With the polariser and analyser in crossed position, turn on the lightsource and place the gemstone on the rotating platform just above the polarizer (this platform may not always be present in which case you use your tweezers).
Observe the gemstone through the analyser and slowly turn the stone, which 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 refractve).

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 behaviours should pose no problems for the unexperienced user. The latter (ADR) can be interpreted as the stone being double refractive.

Video.png Anomalous Double Refraction video
Video showing the behaviour of double refractive stones and ADR under the polariscope - WMV/video format - 5480KB

A possible solution to overcome the confusion when one suspects ADR is to orientate the stone in its lightest position and then quickly turn the analyser 90°. If the stone becomes noticably lighter, the gemstone is single refractive and the stone exhibits ADR. If it stays more or less the same, it is double refractive.

Especially red stones that are out of the limit of the refractometer (OTL) may be difficult to distinguish with the polariscope due to ADR. Some stones in that category are Ruby, red Spinel and red Garnets.

Note: It should be noted that the gem at hand should be transparent to translucent so light can pass through it. If you would put a floortile under the polariscope it would remain dark, which doesn't mean the tile is single refractive.

Advanced

Conoscopy

Expert

Retardation

Sources

  • Ruby & Sapphire (1997) - Richard W. Hughes
  • Gem Identification Made Easy 3th edition (2006) - Antoinette Matlins, A.C. Bonanno

External links