What Are Gemstone Absorption Spectra?

Color is an obvious quality of a gem but alone it tells little about a gem. By using a spectroscope to reveal gemstone absorption spectra, a gemologist can take a fuller view of how gems interact with light and get their color. Determining which wavelengths of the spectrum of light a particular gemstone absorbs is not only useful for identification, it can also be used to determine if a gem is natural or synthetic or if it’s been subjected to coloring treatments, like dyes.

Emerald” is both a gemstone and a term for a shade of green. How can gemologists tell if an emerald colored gemstone is actually an emerald gemstone? “Mackay Emerald” by greyloch is licensed under CC By-SA 2.0

What Does “Absorption” Mean?

In large part, gemstone absorption spectra give gems their color. When white light strikes a gemstone, the photons of particular wavelengths, or colors, are “absorbed” at the atomic level. However, these absorbed photons don’t give the gem color.  These photons effectively become heat.

Although it may seem odd that gems can turn certain colors into heat, there is a well-known example of objects turning colors into heat. Black colored objects absorb all the wavelengths of visible light and convert them into heat. What we see as the color “black” is the absorption of all visible light. White colored objects reflect all wavelengths of visible light, so they get neither heat nor any color from light beyond that of “white light.” (This is why black colored clothes keep you warmer and white colored clothes keep you cooler).

Gemstone light absorption is not the same as the internal reflection of light within a faceted gem that contributes to gemstone dispersion. Light that is absorbed by a gemstone is not trapped inside it bouncing about endlessly. Absorbed light exists only as heat.

The wavelengths of light that are not absorbed by a gem pass through it and create the colors we see with the naked eye. Transparent gems are said to transmit the non-absorbed wavelengths. Opaque gems are said to reflect the non-absorbed wavelengths. Different gem species absorb, transmit, and reflect different wavelengths of light. These combinations are what give them their characteristic colors.

What Do Gemstone Absorption Spectra Look Like?

White light is composed of all visible colors. Its spectrum, when dispersed through a prism, looks like a rainbow. A spectroscope disperses white light and also indicates the wavelengths of colors, usually measured in nanometers (nm) or billionths of a meter.

The visible light spectrum, from a violet color with a wavelength of 380 nm to a red with wavelength of 750 nm. In this image, no colors or wavelengths have been absorbed into any object. “Linear Visual Spectrum – Light,” Public Domain

A spectroscope makes gemstone absorption spectra visible by directing white light into a gem and displaying the resulting rainbow of colors, minus those colors that are absorbed. These absorbed wavelengths of light appear as dark lines. Sometimes entire areas of the spectrum or ranges of colors are absorbed and appear dark.

Gemologists can compare the absorption lines, areas, and patterns they see through a spectroscope with known gemstone absorption spectra to help identify a particular gemstone. Emeralds, for example, will display distinct black absorption lines at 680 and 683 nm, an area of absorption between 580 and 630 nm, and an almost complete absorption of the violet end of the spectrum. Whether or not an emerald looks emerald green, a real emerald will display this absorption spectrum.

emerald absorption spectrum
There are two kinds of spectroscopes, diffraction grating and prism. Both work equally well and measure the same wavelengths. Diffraction grating spectroscopes distribute the wavelengths evenly. Prism spectroscopes distort the blue and red areas in order to make the absorption lines easier to see. “Emerald Absorption Spectrum” by Don Clark

Not all gemstones can be identified by their absorption spectra. Some gems require additional analysis for identification. Nevertheless, observing a gemstone’s absorption spectrum can be a valuable gemological test.