Polariscopes facilitate the rapid detection of common gemstone imitations, such as glasses or synthetic spinels. This is done by studying the optical properties of the stones with polarized light. The polariscope makes it possible to examine both polished and rough stones. It is even possible to examine several stones simultaneously, depending on the size of the polarisers.
How will the examination be conducted? For examination of precious and gemstones – but also syntheses or imitations – these are positioned on the rotating glass plate above the polarizer. Thus, the materials to be examined are located between the polarizer and the analyzer. The lighting comes from below and the materials are viewed from above.
Function of Polariscopes
And this is how the instruments work: The dispersing light oscillates in all directions perpendicular to the direction of propagation. Passing through a polarizer, only one direction of oscillation is remaining. If the polarizer and analyzer are rotated exactly 90 ° against each other for the dark position, the background of the stones is black. This is the dark position (shown figure polariscope left).
What is visible? Very often, glass stones have internal stresses and then show anomalous anisotropy ( direction-dependent polychromy). That means they reveal bright areas when viewed through a polariscope. Dark bars are also typical for glass stones, which move through the stone when turned (center figure). Synthetic spinels produced by the Verneuil process also exhibit anomalous stress birefringence. As cubic substances, they would have to remain dark when rotated under crossed polarisers. The characteristic appearance of the synthetic spinels, the spindle-like brightenings, however, become visible under the polarizer and betray them immediately (Figure on the outer right).
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Video: Getting to know the table-top polariscope for combination with a cold light source
Do you want to read more?
If you are looking for more specialist information, order the white paper “Gemstone Polariscopy”. The publication describes e.g. the following topics:
- Design and functioning of a polariscope
- Demonstration of bright positionand dark position
- Distinguishing ruby, red spinel, red garnet, tourmaline
- Determining gemstone imitations, such as glass or synthetic spinels
- Observation of faceted red stones between crossed polarizers in dark position
- Observation of faceted red stones between crossed polarizers embedded in immersion liquid
- Determining axial images of e.g. the amethyst with an additional conoscopic lens
The whitepaper was produced in collaboration with Prof. Dr. Jochen Schlueter wrote. He is a graduate mineralogist and responsible for the display collection of the Mineralogical Museum in Hamburg.
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