---
title: "Blink (polariscope): meaning in gemology"
description: "A stone blinks in a polariscope when it turns light and dark four times per turn between crossed polarizers. Blinking shows that the stone is doubly refractive."
url: https://digitalgemology.com/learn/glossary/blink-polariscope
image: https://digitalgemology.com/img/og-digital-gemology.png?v=d224fb01
---

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

# Blink (polariscope)

Also called blinks, extinction positions

A stone blinks in a polariscope when it turns light and dark four times per turn between crossed polarizers. Blinking shows that the stone is doubly refractive.

## In more detail

The dark positions come every 90°, where the stone's vibration directions line up with the filters. Looked at down its optic axis, a doubly refractive stone does not blink. This “blink” is not the blink ray class of our light census.

## How we measure it

Our Polariscope calls a stone blinking when its darkest position over a full turn is under half its lightest. It says a stone “stays dark” when its lightest position stays under Y = 0.003. These thresholds are our own choice.

## Related terms

- [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.
- [Doubly refractive](https://digitalgemology.com/learn/glossary/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.
- [Strain](https://digitalgemology.com/learn/glossary/strain): Strain is internal stress that makes a singly refractive stone weakly doubly refractive in patches. Between crossed polarizers it shows as patchy light that never goes dark all at once.
- [Optic axis](https://digitalgemology.com/learn/glossary/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.
- [Census ray classes](https://digitalgemology.com/learn/glossary/census-ray-classes): In our light census, every ray that enters a stone ends in one of five classes. The classes are useful, blink, leak, grazing and lost.

## 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)