The polariscope
The gemstone sets what you see; the shape sets the path through it and the drawing.
Drag the ring, or press [ and ]. A doubly refractive stone goes dark every 90°.
Turned 45°: light.
Ruby is doubly refractive: it splits the light into two rays vibrating at right angles, the lines on the stone. Lined up with the polarizers, at 0° and 90°, it passes nothing; at 45° it is lightest. The two rays differ by many wavelengths over the path, so the light is white (high-order white), tinted by the stone's own color.
0° looks straight down the optic axis; 90° looks across it. Or drag the stone round.
The lower polarizer lets through light vibrating one way only. A singly refractive stone passes it on unchanged, so the crossed upper polarizer stops it. A doubly refractive stone splits it into two rays vibrating at right angles, and the upper polarizer passes part of each, except when the stone's vibration directions line up with the polarizers.
Uniaxial negative: refractive indices 1.770 (o-ray) and 1.762 (e-ray), birefringence 0.008. Path 5.8 mm.
The birefringence is the Refractometer’s, and the tilt the Dichroscope’s: from straight down the optic axis to across it.
What a polariscope shows for each gemstone
| Gemstone | Optics | Birefringence | Crossed polarizers, in the trade | Conoscope figure | This model |
|---|---|---|---|---|---|
| Diamond (shown above) | Isotropic | None | Stays dark (strain can show as patchy light) | None | Stays dark at every turn and tilt |
| Moissanite (shown above) | Uniaxial positive | 0.043 | Blinks; nearly dark face-up when the table is cut across the optic axis | Uniaxial, tight rings | Blinks across the optic axis; stays dark down it; the red plate reads it positive (+) |
| Cubic Zirconia (shown above) | Isotropic | None | Stays dark | None | Stays dark at every turn and tilt |
| Ruby (shown above) | Uniaxial negative | 0.008 | Blinks | Uniaxial | Blinks across the optic axis; stays dark down it; the red plate reads it negative (−) |
| Sapphire (shown above) | Uniaxial negative | 0.008 | Blinks | Uniaxial | Blinks across the optic axis; stays dark down it; the red plate reads it negative (−) |
| Emerald (shown above) | Uniaxial negative | 0.006 | Blinks | Uniaxial | Blinks across the optic axis; stays dark down it; the red plate reads it negative (−) |
| Amethyst (shown above) | Uniaxial positive | 0.009 | Blinks | Bull's-eye: no cross at the center | Blinks across the optic axis; stays light down it, in color; the red plate reads it positive (+) |
Optics and birefringence are the simulator’s, the same numbers as the Refractometer shows. This model: a turn across the optic axis, then straight down it, between crossed polarizers, through the hexahedron.
A model of a gem polariscope: one straight path through each gemstone in daylight, between ideal polarizers, computed by Jones calculus from its refractive indices and measured absorption spectra. It is not a test of a real stone. Most screens give out polarized light, so never test a real polariscope against one. How the polariscope is modeled.