---
title: "Optic sign: meaning in gemology"
description: "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…"
url: https://digitalgemology.com/learn/glossary/optic-sign
image: https://digitalgemology.com/img/og-digital-gemology.png?v=d224fb01
---

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[Glossary](https://digitalgemology.com/learn/glossary) · Optics

# Optic sign

Also called optically positive, optically negative

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](https://digitalgemology.com/learn/glossary/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](https://digitalgemology.com/learn/glossary/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](https://digitalgemology.com/learn/glossary/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](https://digitalgemology.com/learn/glossary/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.

## Learn more

- [Polariscope](https://digitalgemology.com/about/how-we-measure/polariscope)
- [Polariscope](https://digitalgemology.com/tools/polariscope)

[All 328 terms](https://digitalgemology.com/learn/glossary)