Polariscope

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Revision as of 03:52, 25 September 2006 by Doos (talk | contribs) (Conoscopy)
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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.

Anisotrope gemstones can have one or two direction(s) in which it will stay dark throughout lateral rotation. These directions are the optic axes of the gemstone. No double refraction occurs along those directions. Uniaxial stones have one optic axis whilst biaxial gemstones have two optic axes.
Therefor it is useful to view the stone under a different angle when it stays dark as a confirmation.

Advanced

Conoscopy

Conoscope.jpg

In gemology we use a conoscope (a strain free acrylic or glass sphere on a rod) to determine optic character (uniaxial or biaxial) in anisotrope gemstones. The conoscope creates a 2-dimensional image of the 3-dimensional interference in a mineral.
Although determining the optic character with a conoscope is a fairly easy procedure, finding the interference itself figure is not. The interference figures always appear on the optic axes of minerals.

The simplest way to find an interference figure is to rotate the stone under the polariscope in every possible direction, while looking down the analyser, until one sees a small flash of colours appear on the surface of the gemstone. When that flash of colours is found, fix the stone in that position and hoover your conoscope slightly over it. Now, while still looking through the analyser, you should see the colour flash transform into a rounded 2-dimensional image.
This image will appear different in uniaxial stones than in biaxial stones, each with their characteristic pattern.

Due to enantiomorphism Quartz will give a typical uniaxial image but with a large gap in the middle. That is what is named a "bull's eye" and is typical for twinned Quartz (both natural as synthetic).

Because anisotropic minerals when viewed down the optic axis, another technic for finding the optic axis can be used. Turn the stone under the polariscope from all sides and see where the gemstone does not blink light and dark on lateral rotation. That will be the optic axis.
Remember that uniaxial minerals have one optic axis while biaxial gemstones have two optical axes.



The above typical images may not be seen in whole at times, but don't be alarmed, one can determine optic character from part of the conoscopic image.

Retardation

Sources

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

External links