The dichroscope
The gemstone sets the colors; the shape only changes the drawing.
Drag the ring, or press [ and ]. At 45° the windows match; at 90° they swap.
Two colors: the o-ray (ordinary), vibrating across the optic axis, is purplish red; the e-ray (extraordinary), vibrating along it, is orangy red. Ruby absorbs the two differently, so it is dichroic.
0° looks straight down the optic axis; 90° looks across it.
The stone splits white light into two rays vibrating at right angles: the o-ray (ordinary) always vibrates across the optic axis, the e-ray (extraordinary) in the plane of the axis. The calcite sends each direction of vibration to its own window.
Both rays measured: GIA's published Cr3+ cross-sections (Dubinsky, Stone-Sundberg & Emmett 2020), at the chromium content of a natural ruby from Thailand (G.R. Rossman, Caltech).
What a dichroscope shows for each gemstone
| Gemstone | Optics | Dichroism in the trade | o-ray and e-ray colors | This model, ΔE00 |
|---|---|---|---|---|
| Diamond (shown above) | Isotropic | None | None | 0.0, none |
| Moissanite (shown above) | Uniaxial positive | None visible (colorless) | None | 0.0, none |
| Cubic Zirconia (shown above) | Isotropic | None | None | 0.0, none |
| Ruby (shown above) | Uniaxial negative | Strong | Purplish red and orangy red | 17.9, strong |
| Sapphire (shown above) | Uniaxial negative | Strong | Violetish blue and greenish blue | 41.6, strong |
| Emerald (shown above) | Uniaxial negative | Distinct | Yellowish green and bluish green | 15.3, distinct |
| Amethyst (shown above) | Uniaxial positive | Weak to moderate | Bluish purple and reddish purple | 3.3, weak |
ΔE00 is the CIEDE2000 color difference between the two windows, across the optic axis with the dichroscope lined up, through 5 mm in daylight. Under 1 reads as none, 1 to 5 weak, 5 to 16.5 distinct, over 16.5 strong.
A model of a calcite dichroscope: a straight 5 mm path through each gemstone in daylight (D65), with its color computed from measured polarized absorption spectra. It is not a test of a real stone. Most screens give out polarized light, so never test a real dichroscope against one. How the dichroscope is modeled.