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.
In more detail
Tanzanite and iolite are biaxial. On a refractometer, both shadow edges of a biaxial stone can move as the stone turns.
How we measure it
None of the seven gemstones in our tools is biaxial, so the tools do not model biaxial stones.
Related terms
- Uniaxial: A uniaxial stone is a doubly refractive stone with one optic axis. Moissanite, emerald, ruby, sapphire and amethyst are uniaxial.
- 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.
- Pleochroism: 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.
- Interference figure: An interference figure is the pattern of colors and dark bands that a doubly refractive stone shows through a conoscope. Each point of the figure is one direction through the stone.