A doubly refractive stone splits light into two rays that vibrate at right angles and travel at different speeds. Each ray has its own refractive index.
In more detail
Moissanite, ruby, sapphire, emerald and amethyst are doubly refractive. In moissanite, the split is large enough to double the facet edges you see through the stone. Along an optic axis, light does not split.
How we measure it
The simulator's render traces both rays in doubly refractive stones. The Refractometer shows two edges where the stone is within range, and the Polariscope shows it blink.
Related terms
- Birefringence: Birefringence is the difference between the two refractive indices of a doubly refractive stone. It shows how strongly the stone splits light into two rays.
- Optic axis: An optic axis is a direction in a doubly refractive crystal along which light does not split into two rays. Seen along it, the stone behaves as if it were singly refractive.
- 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.
- Facet doubling: Facet doubling is the double image of facet edges seen through a strongly doubly refractive stone. It shows clearly in moissanite under a 10× loupe.