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

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

# Conoscope

Also called conoscope sphere

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.

## In more detail

With the figure, a gemologist can tell a uniaxial stone from a biaxial one. With a first-order red plate, the figure also gives the optic sign.

## How we measure it

Our Polariscope's conoscope maps a 40° cone of directions in air. The 40° is a working choice, not a measured sphere.

## Related terms

- [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.
- [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.
- [Optic sign](https://digitalgemology.com/learn/glossary/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.
- [Polariscope](https://digitalgemology.com/learn/glossary/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.

## Learn more

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

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