Unlocking the Secrets of Gemstones with a Polariscope

A polariscope is an essential tool for gemologists, utilising polarised light to identify various optical properties of gemstones. This instrument consists of two polarised filters—one at the top (analyzer) and one at the bottom (polarizer). When these filters are aligned at right angles to each other, the field between them remains dark, a setup known as the "crossed position." This arrangement helps in determining whether a gem is isotropic, anisotropic, anomalously double refractive, or an anisotropic aggregate.

How to Operate a Polariscope

1. Set Up the Polariscope: Ensure the polarizer and analyzer are in the crossed position.
2. Place the Gemstone: Position the gemstone on the rotating platform above the polarizer or use tweezers if a platform is not available.
3. Observe the Stone: Look through the analyzer and slowly rotate the gemstone. Based on the behaviour of the stone under polarised light, you can identify its optical properties.

Possible Observations and Interpretations

1. Stone Remains Dark: If the stone stays dark throughout a 360° rotation, it is isotropic (single refractive).
2. Stone Blinks Four Times: If the stone alternates between light and dark four times during a 360° rotation, it is anisotropic (double refractive).
3. Stone Remains Light: If the stone stays light throughout the rotation, it is likely a microcrystalline or cryptocrystalline aggregate (e.g., chalcedony).
4. Anomalous Double Refraction (ADR): If the stone shows ADR, it appears light continuously. This indicates it is isotropic but mimicking double refraction, which can be confusing.

For ADR, to differentiate between a true double refractive stone and one exhibiting ADR, orient the stone in its lightest position and quickly turn the analyzer 90°. If the stone becomes noticeably lighter, it is single refractive with ADR. If it remains the same, it is double refractive.

Advanced Techniques: Conoscopy

A conoscope, a sphere on a rod, is used with a polariscope to determine the optic character (uniaxial or biaxial) of anisotropic gemstones. By rotating the stone under the polariscope and looking for flashes of colour, you can position the conoscope to view a 2D interference figure. Uniaxial stones produce distinct patterns different from biaxial stones.

Finding the Optic Axis:

• Rotate the stone under the polariscope and observe from different angles.
• When the gemstone does not blink during lateral rotation, you are viewing it along its optic axis.
• Uniaxial minerals have one optic axis, while biaxial minerals have two.

Turning a Gemological Microscope into a Polarising Microscope

You can convert a gemological microscope into a polarising microscope by placing a polarising sheet over the light source and another one in the crossed position below the optics. This setup, which can be achieved for less than $30, helps distinguish between solid and negative crystal inclusions and other internal features of gemstones.

Practical Applications

• Isotropic Stones: Remain dark during full rotation.
• Anisotropic Stones: Blink four times.
• Aggregates: Remain light continuously.
• ADR Detection: Use the 90° analyzer rotation test.
• Conoscopy: Determine uniaxial or biaxial nature using interference patterns.

Conclusion

A polariscope is a versatile and powerful tool in gemology, enabling the identification of various optical characteristics in gemstones. With practice and a keen eye, you can unlock the secrets hidden within your gems, ensuring accurate identification and appreciation of their unique properties.