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.
In more detail
Moissanite's birefringence is 0.043, amethyst's 0.009, ruby's and sapphire's 0.008 and emerald's 0.006. Diamond and cubic zirconia have none. On a refractometer, the largest gap between the two shadow edges gives it.
How we measure it
The Refractometer shows two edges for stones within its range. The Polariscope uses it to compute how a stone lights up between crossed polarizers. The simulator's render traces both indices, but every score uses a single index.
Related terms
- Doubly refractive: 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.
- 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.
- Refractometer: A refractometer is a gem-testing instrument that reads the refractive index of one polished facet. The facet lies on a drop of contact liquid on a glass of higher index.
- Polariscope: A polariscope is a gem-testing instrument with a stone between two polarizing filters. With the filters crossed, it shows whether a stone is singly refractive, doubly refractive, strained or an aggregate.