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.
In more detail
In uniaxial stones, the optic axis runs along the crystal's c-axis. Cutters often set it square to the table: in moissanite this hides the doubling face-up. In ruby and sapphire, this orientation gives the best face-up color. So look at a stone from more than one direction before you call it singly refractive.
How we measure it
The simulator and the Refractometer set the optic axis square to the table. The Polariscope and Dichroscope let you tilt the stone from straight down the axis to across it. The Spectroscope looks across the axis.
Related terms
- Uniaxial: A uniaxial stone is a doubly refractive stone with one optic axis. Moissanite, emerald, ruby, sapphire and amethyst are uniaxial.
- 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.
- 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.
- 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.
- 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.