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How to use the polariscope
To use the polariscope the top polarizing filter is turned to the extinction position (dark) allowing the least amount of light to pass through the two filters.
The stone is placed between the two filters and rotated around a vertical axis.
If it darkens evenly at exactly 90o intervals (4x per rotation) it indicates double refraction. It is important to test each stone in more than two positions as a doubly refractive stone may have one or two directions of single refraction (optic axes) in it.
If it remains dark throughout its rotation single refraction is indicated. Cryptocrystalline material remains bright during rotation (jade, chalcedony) but some glass with textured backs does the same so it is best to use this test for transparent stones only. Note that large numbers of doubly refractive inclusions would have a similar effect. Brilliant cut stones should not be tested table up or down as they may reflect out all the light entering the table giving a false reading.
Anomalous double refraction may occur in singly refractive materials with internal strains such as diamond, garnet, synthetic spinel, amber, plastic, opal and glass. The extinction patterns here however do not occur at precisely 90o intervals although rarely they may be close. Plastic and amber may show bright interference colours. Glass may show a characteristic cross like shadow, or two approaching bars which almost form a cross during rotation. Synthetic spinel shows a characteristic 'tabby' extinction, a sort of fine, mottled cross hatching of parallel silk-like lines which change during rotation. In a doubly refractive stone interference colours will appear when one is within a few degrees of an optic axis.
Instructions provided by Charles Lewton-Brain.