---
title: "Guide minimum pavilion angle: meaning in gemology"
description: "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…"
url: https://digitalgemology.com/learn/glossary/guide-minimum-pavilion-angle
image: https://digitalgemology.com/img/og-digital-gemology.png?v=d224fb01
---

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

# Guide minimum pavilion angle

Also called 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.

## In more detail

It is 38.9° for diamond, 39.5° for moissanite and 38.1° for cubic zirconia. It is 36.4° for ruby and sapphire, 39.4° for emerald and 40.4° for amethyst. Faceters usually cut a degree or two steeper, for light that arrives off the axis.

## How we measure it

Each gemstone page gives the figure for a round design, from the lower end of the stone's refractive index. Fancy shapes vary, so every cut page has its own head-on test.

## Related terms

- [Critical angle](https://digitalgemology.com/learn/glossary/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).
- [Pavilion angle](https://digitalgemology.com/learn/glossary/pavilion-angle): The pavilion angle is the slope of the pavilion mains, measured against the plane of the girdle.
- [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)

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