The spectroscope is a fundamental tool in gemology used for examining which parts of white light are absorbed by a gemstone or other materials. As white light passes through or reflects off a gem, specific wavelengths are absorbed within the visible spectrum, leaving behind dark lines or bands that serve as an optical fingerprint for identification.
There are two primary types of spectroscopes used in gemological analysis:
- Diffraction Grating Spectroscopes: Based on light diffraction.
- Prism Spectroscopes: Based on light dispersion through a series of glass prisms.
Light Absorption Fundamentals
Visible white light consists of seven primary spectral colors: Red, Orange, Yellow, Green, Blue, Indigo, and Violet. Each color travels at a specific wavelength and speed. When white light enters a gemstone, certain components are absorbed by the elements within the crystal structure, while the remaining wavelengths determine the perceived color of the gem.
For instance, if a gemstone absorbs all colors of the visible spectrum except red, only the red light reaches the eye, making the stone appear red. Similarly, if all colors except red and blue are absorbed, the resulting residual light produces a purple hue.
In actual gemological testing, spectrum images are highly detailed, showing precise absorption lines and bands. The absorbed light energy transforms inside the gemstone into heat energy.
Types of Spectroscopes in Gemology
1. Diffraction Grating Spectroscope
This type uses a thin film with closely spaced parallel lines (grating material) to diffract light into a linear spectrum. It produces an even, wide spread across all wavelengths, providing an expanded view of the red region compared to prism spectroscopes.
2. Prism Spectroscope
Prism spectroscopes rely on dispersion through a train of prisms. Light enters through an adjustable slit and is spread into a rainbow. Because dispersion is non-linear, the blue area of the spectrum appears more spread out, while the red end is more condensed.
How to Use a Spectroscope Correctly
To obtain clear absorption spectra, proper lighting and stone placement are essential:
- Reflected Light Method: Direct white light onto the pavilion of the gemstone at a 45° angle. Position the spectroscope at the opposite 45° angle to capture reflected light traveling through the longest internal path of the stone.
- Non-Reflective Background: Place the stone on a black, non-reflective surface (such as black velvet) to avoid false readings from background light.
- Transmitted Light Method: Hold the gemstone directly between a strong cold-light penlight and the spectroscope slit to observe light passing straight through.
Mastering the spectroscope takes practice! Explore more gemological instrument guides and identification techniques on Horizon Gems.
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