Pleochroism is the property of a colored doubly refractive stone to show different colors in different directions. It happens because the stone absorbs its rays differently.
In more detail
A uniaxial stone can show two colors (dichroism); a biaxial stone can show three (trichroism). Ruby shows purplish red and orangy red; sapphire violetish blue and greenish blue. Emerald shows yellowish green and bluish green. Colorless doubly refractive stones, such as moissanite, show no pleochroism.
How we measure it
The Dichroscope computes the two colors from measured polarized absorption spectra. It classes the color difference ΔE00 as none (under 1), weak (1 to 5), distinct (5 to 16.5) or strong. The simulator's pleochroism is tuned to each stone's published pair, not measured.
Related terms
- Dichroscope: A dichroscope is a gem-testing instrument that shows a stone's two pleochroic colors side by side. In the calcite type, a calcite rhomb splits the light into two images of a small window.
- Ordinary ray: The ordinary ray (o-ray) is the ray in a uniaxial stone that vibrates across the optic axis. It has the same refractive index, ω or nₒ, in every direction.
- Extraordinary ray: In a uniaxial stone, the extraordinary ray (e-ray) vibrates in the plane of the optic axis and the light path. Its refractive index changes with direction, from ω along the axis to ε across it.
- CIEDE2000 color difference: CIEDE2000 (ΔE00) is a standard formula for how different two colors look to a human observer. A larger number means a larger visible difference.
- Biaxial: A biaxial stone is a doubly refractive stone with two optic axes. It can show three colors in a dichroscope, and its interference figure differs from a uniaxial one.