---
title: "Facet doubling: meaning in gemology"
description: "Facet doubling is the double image of facet edges seen through a strongly doubly refractive stone. It shows clearly in moissanite under a 10× loupe."
url: https://digitalgemology.com/learn/glossary/facet-doubling
image: https://digitalgemology.com/img/og-digital-gemology.png?v=d224fb01
---

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

# Facet doubling

Also called doubled facet edges, doubling, loupe test

Facet doubling is the double image of facet edges seen through a strongly doubly refractive stone. It shows clearly in moissanite under a 10× loupe.

## In more detail

Look through a crown facet at an angle, not straight down through the table. Along the optic axis, nothing doubles. Doubling proves that a stone is not diamond. It does not prove moissanite, because other doubly refractive stones double too.

## How we measure it

The 3D Gemstone Simulator traces both rays, so moissanite's edges double in its render at an angle.

## Related terms

- [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.
- [Birefringence](https://digitalgemology.com/learn/glossary/birefringence): Birefringence is the difference between the two refractive indices of a doubly refractive stone. It shows how strongly the stone splits light into two rays.
- [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.
- [3D Gemstone Simulator](https://digitalgemology.com/learn/glossary/3d-gemstone-simulator): The 3D Gemstone Simulator is a free Digital Gemology tool that renders a cut stone as a camera sees it. You choose the cut, gemstone, carat weight, color, clarity, cut grade and lighting.

## Learn more

- [The loupe test: doubled facet edges in moissanite](https://digitalgemology.com/learn/diamond-vs-moissanite/moissanite-doubled-facets-loupe-test)
- [3D Gemstone Simulator](https://digitalgemology.com/about/how-we-measure/simulator)

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