---
title: "Critical angle: meaning in gemology"
description: "The critical angle is the angle from a facet's normal beyond which light inside a stone reflects totally. It follows from the refractive index: θc =…"
url: https://digitalgemology.com/learn/glossary/critical-angle
image: https://digitalgemology.com/img/og-digital-gemology.png?v=d224fb01
---

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

# Critical angle

The critical angle is the angle from a facet's normal beyond which light inside a stone reflects totally. It follows from the refractive index: θc = arcsin(1/n).

## In more detail

A higher index gives a smaller critical angle. Diamond's is 24.4°, moissanite's 22.2°, cubic zirconia's 27.7°, ruby's and sapphire's 34.6°, emerald's 39.4° and amethyst's 40.4°. A refractometer uses a different critical angle: the one in its glass, against the stone.

## How we measure it

Our gemstone pages give each stone's critical angle from the lower end of its refractive index. The head-on test on each cut page checks which pavilion facets lie at or above it.

## Related terms

- [Refractive index](https://digitalgemology.com/learn/glossary/refractive-index): Refractive index is the speed of light in a vacuum divided by its speed in the stone. A higher index bends light more where it crosses a surface at an angle.
- [Total internal reflection](https://digitalgemology.com/learn/glossary/total-internal-reflection): Total internal reflection happens when light inside a stone meets a facet beyond the critical angle. The facet then acts as a perfect mirror, and all the light reflects back into the stone.
- [Guide minimum pavilion angle](https://digitalgemology.com/learn/glossary/guide-minimum-pavilion-angle): The guide minimum pavilion angle is the shallowest pavilion at which a round still returns head-on light through the table. It is the larger of the critical angle and (180° − critical angle) ÷ 4.
- [Head-on light](https://digitalgemology.com/learn/glossary/head-on-light): Head-on light is light that enters a stone straight down along its axis. Our head-on test gives the share of the pavilion whose facets are steep enough to reflect that light.
- [Windowing](https://digitalgemology.com/learn/glossary/windowing): A window is a face-up area through which you can see straight through a stone. It forms where pavilion facets are too shallow to reflect light, so light passes out of the back.

## Learn more

- [Gemstone constants](https://digitalgemology.com/about/how-we-measure/gemstone-constants)
- [Diamond 3D models and cutting files](https://digitalgemology.com/gemstones/diamond)
- [Refractometer](https://digitalgemology.com/about/how-we-measure/refractometer)

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