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

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

# Refractive index

Also called RI, index of refraction

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.

## In more detail

Diamond's index is 2.417. Moissanite reads 2.648 to 2.691 and cubic zirconia 2.15 to 2.18. Ruby and sapphire read 1.762 to 1.770, emerald 1.577 to 1.583 and amethyst 1.544 to 1.553. The index sets the critical angle, so a design for diamond does not work the same way in another stone.

## How we measure it

Our gemstone pages give each stone's index at the sodium line, 589 nm. For angle limits we use the lower end of a range, because it gives the stricter limit. On the Refractometer, diamond, moissanite and cubic zirconia read over the limit.

## 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).
- [Dispersion](https://digitalgemology.com/learn/glossary/dispersion): Dispersion is how differently a stone bends red and violet light. Gemologists give it as the index difference between the B line (686.7 nm) and the G line (430.8 nm).
- [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.
- [Refractometer](https://digitalgemology.com/learn/glossary/refractometer): A refractometer is a gem-testing instrument that reads the refractive index of one polished facet. The facet lies on a drop of contact liquid on a glass of higher index.
- [Snell's law](https://digitalgemology.com/learn/glossary/snells-law): Snell's law sets the direction of refracted light: n₁ sin θ₁ = n₂ sin θ₂. The angles are measured from the normal to the surface.

## Learn more

- [Gemstone constants](https://digitalgemology.com/about/how-we-measure/gemstone-constants)
- [Moissanite's refractive index, and why it needs its own cut](https://digitalgemology.com/learn/diamond-vs-moissanite/moissanite-refractive-index-and-cut)
- [Refractometer](https://digitalgemology.com/about/how-we-measure/refractometer)

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