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.
In more detail
Its center, the melatope, marks the optic axis. A uniaxial stone seen down its axis shows a dark cross centered on the melatope, often with colored rings. Quartz, such as amethyst, shows a bull's-eye with no cross at the center, because of its optical activity. A biaxial stone shows a different figure.
How we measure it
The Polariscope computes the figure by Jones calculus over a 40° cone of directions. The melatope leaves the view when the stone tilts past 14° (moissanite) to 25° (amethyst).
Related terms
- Conoscope: A conoscope is a strain-free glass sphere set on a stone in a polariscope. It gathers light from many directions at once, so the eyepiece shows an interference figure.
- 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.
- Optic sign: The optic sign of a uniaxial stone tells which of its two rays is slower. It is positive when the extraordinary index is higher than the ordinary one, and negative when it is lower.
- First-order red plate: A first-order red plate is a retarder in a polariscope that shows which of a stone's two rays is slower. It turns parts of the interference figure blue or yellow, and that gives the optic sign.
- Optical activity: 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.