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.
In more detail
Ruby's chromium R lines are a close pair at 692.8 and 694.2 nm. Sapphire's strongest iron line is at 450 nm. Emerald shows chromium lines at 683.0 and 680.5 nm. A yellow (Cape) diamond shows its N3 line at 415.5 nm.
How we measure it
The Spectroscope names each line and band and classes it by contrast. A line under 0.04 is too faint to see, under 0.15 weak, under 0.4 distinct, and strong above that. Bands use the steps 0.1, 0.35 and 0.7. These steps are our own choice, not a published scale.
Related terms
- Absorption spectrum: An absorption spectrum shows which wavelengths of light a stone absorbs. In a spectroscope, the absorbed parts of the spectrum look dark.
- 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.
- 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.