Testing of Gemstones for Identification through Polariscope

polariscope is an instrument in gemology for testing of gems with single refraction, double refraction, Poly-crystalline material or ADR.

Let us first tell you what is Polariscope? and how test is done?. The name of Polar – Scope itself shows or describes that a method to see the stones from poles or axis through the Scope. It helps in understanding different colour of light being transmitted by a gemstone. Polariscope basically is an optical device. A polariscope is composed chiefly of a light source and two crossed polarized lenses such as Polaroid {{Registered Trademark}}. One can determine both optic character and the optic sign of a gemstone. It helps in understanding synthetic, glass or natural gemstones.

In Polariscope, there are two lenses. One is on the top side called ” Analyser”. The other part is at the bottom which is called ” Polariser”. Above Polariser, there is ” Tint  Plate “.  Stone is put on the polariser and analyse through Analyser. Tint Plate plays role in rotation so gems view can be viewed by 360 degree.

How to Analyse through Polariscope:

There are brief description of two phase testing methods by polariscope that how gems are tested.

1st Phase of Test:-

Step:1 Rotate the analyser and check if there is complete darkness

Step: 2 Put gem on polariser

Step: 3 move the gem or rotate the Tint Plate at 360 degree and note down the light refraction spectrum on gems.

2nd Phase of Test for Further Confirmation:-

Step:1 Rotate the analyse and check if there is complete darkness

Step: 2 Put gem on polariser

Step: 3 Leave the gem in full light condition

Step: 4 now quickly rotate the analyser only one time and see final condition of light on gem.

Type of Identifications of Gems through Polariscope

By the above test, following results can be found for Gemstones that in what following categories they fall in:-

  • SR ( Single Refraction) / isotropic / Cubic crystalline
  • DR (Double Refraction) Anisotropic
  • ADR (Anomalously double refractive or an
  • Anisotropic Aggregate (Polycrystalline)

1- SR ( Single Refraction) / isotropic / Cubic crystalline

Observations:

Stone remains dark through 360 Degree rotation of Tilt Plate. Single Refractive of Isotropic gemstones gives constant dark light. In 2nd phase test, if gemstone appears bright light, it is SR. It occurs in mostly non crystalline gemstones.

Conclusions

Isotropic – amorphous or cubic

Examples

garnet, spinels, paste, natural glass, flourite, diamond, opal, plastics

2- DR (Double Refraction) Anisotropic

Observations:

Stone becomes light and dark four times through each 360 degree rotation in most orientation. In double refractive, gemstones gives changing light pattern. Sometime it appears as dark and some time it appears as light. This happens with mostly in crystalline gemstones.

Conclusions

Anisotropic – uniaxial  or biaxial

Examples

Beryl, Chrysoberyl, Corundum, feldspar, peridot, quartz, topaz,, tourmaline, zircon, tanzanite

  

3-ADR (anomalously double Refractive)

Observations:

Stone shows anomalous extinction effect. In this case, the gemstone gives anomalously double refractive. The pattern of the gems gives snake like dark and light both refraction. Smooth snake like refraction occurs mostly in Synthetic or glass stones.

Conclusions

Strain Anisotropy, often in otherwise isotropic material.

Examples

Almandine garnet, diamond, paste, natural glass, verneuil, synthetic spinel, some plastic, amber and some fire opals,

4-Anisotropic Aggregate (Polycrystalline)

Observations:

In this case, gemstone gives only bright light. Stone stays light through 360 degree rotation in all orientations.

Conclusions

Polycrytalline. Some twinned crystals, some doublets and triplets. Anomalous internal reflection effects.

Examples

Jadite, Nephrite, Chalcedony / Agate, Twinned sapphire, sapphire / synthetic ruby doublet, Reflection from cleavage in flourite.

Polariscope Tutorial by Nick Hodgson of Astrogems

 

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