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: 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: 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: 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: 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: 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.