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.
In more detail
Moissanite and amethyst are uniaxial positive. Ruby, sapphire and emerald are uniaxial negative. A gemologist reads the sign with a conoscope and a first-order red plate.
How we measure it
The Polariscope reads the sign from the colors of its computed interference figure. Blue in the northeast quadrant means positive; blue in the northwest means negative.
Related terms
- Uniaxial: A uniaxial stone is a doubly refractive stone with one optic axis. Moissanite, emerald, ruby, sapphire and amethyst are uniaxial.
- 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.
- 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.
- 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.