From The Gemology Project
Revision as of 02:40, 12 September 2006 by Doos (talk | contribs)
Jump to: navigation, search

The spectroscope is a tool to examin which parts of white light are absorbed by a gemstone (as well as in other materials).
Materials can absorb parts of the electromagnetic spectrum and when the absorbed parts fall with in the visible range, that absorbed part will influence the color of the material.
When a gemstone is observed with a spectroscope, the absorbed parts show as dark lines and/or bands in the spectroscope image.

OPL teaching diffraction grating spectroscope on stand (left) and prism spectroscope with adjustable slit (right)

There are two types of spectroscopes used in gemology:

  1. Diffraction grating spectroscopes (based on diffraction)
  2. Prism spectroscopes (based on dispersion)



Color as perceived by the human eye consists of the 7 colors of the rainbow: Red, Orange, Yellow, Green, Blue, Indigo and Violet. All these colors travel at different speeds and have their own wavelengths. When all the above colors combine, we see it as white light.

When white light reaches a substance, part of the light components may be absorbed by the substance, the other components (residu) form the color of that substance. For instance if a gemstone would absorb all the colors of the rainbow except red, only the red part of the original white light will be visible and the gemstone will therefor be red.
When viewed through a spectroscope, the absorbed parts will disappear from the spectrum image and only red will be visible in the prism of the spectroscope.
Likewise if all colors except red and blue are obsorbed by a gemstone, the residual colors (red and blue) will give rise to a purple gemstone.

The pictures below give a crude example of both above mentiond situations.

absorption of all wavelengths except red
absorption of all wavelengths except red and blue

Of course in real life the spectrum images are much more sophisticated with small lines and bands indicating specific absorption parts of white light.
The energy from the absorbed colors (or better "wavelengths") is transformed inside the gemstone into other types of energy, mostly heat.

One should consider color as a form of energy traveling at a specific wavelength.

Types of spectroscopes

In gemology we make use of two different types of spectroscopes each with their own characteristics.

1. Diffraction grating spectroscope

The Diffraction grating specroscope is based on the principle of diffraction. Maybe the best known brand is OPL, which is produced in the UK by Colin Winter.
Light enters through a narrow slit and is then diffracted by a thin film of diffraction grating material. This produces a linear spectrum image with a generally larger view of the red part than a prism spectroscope.
These spectroscopes do not have a built in scale.

diffraction grating spectrum
Inside the diffraction grating spectroscope
diffraction grating spectrum with scale in nm

2. Prism spectroscope

The prism spectroscope is based on dispersion. The light enters through a narrow slit (some models allow you to adjust the width of the slit) and is then dispered through a series of prisms. Some models have an attachement with a built in scale and these are generally more expensive than their diffraction type cousins.
As these models are based on dispersion, the blue area of the spectrum is more spread out and the red parts are more condensed than the diffraction grating types.

prism spectrum
Inside the prism spectroscope
prism spectrum with scale in nm

Use of the spectroscope

Using the spectroscope poses many problems for those who are not familiar with the instrument. Therefor before attempting to determine the absorption spectra of gems it is best to hold the spectroscope against some different sources of illumination, such as a fluorescent lightbulb, a computer monitor etc. This will show you very clear absorption bands in most cases.

use of the spectroscope with reflected light

Proper use of the spectroscope and lightning is vital when wanting to see good spectra of gemstones.
The most widely used technique is to make use of reflected light. Light enters the pavillion of a gemstones at a 45° angle and the spectroscope should be placed at the same angle on the other side.
The light will travel its longest posible path in this way, picking up the most color.

To prevent the background on which the stone lays to cause falls readings, one should use a black non-reflective underground, such as a small piece of black velvet.

Another technique is to position the gemstone and the lightsource (penlight) in one hand in such a way that the lighsource illuminates the gem from behind. Thus viewing the gemstone in transmitted light.

There are nice spectroscope stands (some with builtin illumination) on the market, but some experience eliminates the need for them.
For the new user it is recommended to start with a gemstone that produces a clear absorption spectrum, such as synthetic Ruby.

Related topics


  • A Students' Guide to Spectroscopy (2003) - Colin Winter FGA, DGA