Polariscope
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Definition
An instrument using two polarizing filters to determine whether a gemstone is singly refractive (isotropic) or doubly refractive (anisotropic), and to detect strain or optical anomalies.
Detailed Explanation
The polariscope consists of a lower polarizer and an upper analyzer set at 90° to each other (crossed polars). An isotropic stone (glass, garnet, spinel) goes to extinction in all positions as the stone is rotated — it appears dark throughout a 360° rotation. An anisotropic stone (quartz, topaz, sapphire) blinks light and dark four times per revolution. Anomalous double refraction — caused by internal strain — appears in normally isotropic gems like diamond and glass under stress, producing irregular interference patterns. Optic figure identification with the polariscope helps distinguish uniaxial from biaxial gems.