Optical activity is the turning of the plane of vibration of light as it passes through some crystals. Quartz, and so amethyst, turns it as light travels along the optic axis.
In more detail
Because of it, a quartz slab cut across its axis stays light, in color, between crossed polarizers. It also gives quartz a bull's-eye interference figure with no cross at the center.
How we measure it
The Polariscope uses ρ(λ) = 7.19 λ² / (λ² − 0.0926283²)² degrees per mm, with λ in µm. That gives 21.8° per mm at 589 nm, after Chandrasekhar (1957).
Related terms
- 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.
- 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.
- Polarized light: Polarized light vibrates in one direction across its path, instead of in every direction. A polarizing filter passes only the light that vibrates along its own direction.
- 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.