---
title: "Absorption spectrum: meaning in gemology"
description: "An absorption spectrum shows which wavelengths of light a stone absorbs. In a spectroscope, the absorbed parts of the spectrum look dark."
url: https://digitalgemology.com/learn/glossary/absorption-spectrum
image: https://digitalgemology.com/img/og-digital-gemology.png?v=d224fb01
---

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

# Absorption spectrum

Also called spectrum

An absorption spectrum shows which wavelengths of light a stone absorbs. In a spectroscope, the absorbed parts of the spectrum look dark.

## In more detail

The pattern often names the coloring element, not the stone: chromium in ruby and emerald, iron in sapphire. A laboratory-grown ruby shows the same chromium lines as a natural one.

## How we measure it

The Spectroscope, Dichroscope and Polariscope compute from measured polarized spectra. Ruby uses GIA's published Cr³⁺ cross-sections (Dubinsky and others, 2020). Sapphire, emerald and amethyst use G.R. Rossman's measured files from Caltech.

## Related terms

- [Absorption line](https://digitalgemology.com/learn/glossary/absorption-line): An absorption line is a narrow dark line in a spectrum, where a stone absorbs a narrow range of wavelengths. An absorption band is a broad dark stretch where it absorbs a wide range.
- [Spectroscope](https://digitalgemology.com/learn/glossary/spectroscope): A spectroscope is a gem-testing instrument that spreads light from a stone into a spectrum. Dark lines and bands show where the stone absorbs.
- [Pleochroism](https://digitalgemology.com/learn/glossary/pleochroism): Pleochroism is the property of a colored doubly refractive stone to show different colors in different directions. It happens because the stone absorbs its rays differently.
- [Beer–Lambert law](https://digitalgemology.com/learn/glossary/beer-lambert-law): The Beer–Lambert law says that the light a stone lets through falls off exponentially with path length: T = e^(−σℓ). Here σ is the absorption coefficient and ℓ is the path length.
- [CIE colorimetry](https://digitalgemology.com/learn/glossary/cie-colorimetry): CIE colorimetry turns a light spectrum into a color by weighting it with standard human color-matching functions. Standard illuminants, such as D65 (daylight) and A (incandescent lamp, 2856 K), define the light.

## Learn more

- [Spectroscope](https://digitalgemology.com/about/how-we-measure/spectroscope)
- [Dichroscope](https://digitalgemology.com/about/how-we-measure/dichroscope)

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