Use of a Spectroscope With Gemstones

A gemmologist has many tools available to identify gemstones.  One of the diagnostic instruments in the spectroscope.  With the use of a flashlight and a small pocket spectroscope a gemologist can identify many common gemstones.

OPL Gemology Spectroscope

Spectroscope Photo by J McKercher

The Visible Spectrum

Visible Spectrum in Cake

Visible Spectrum in Cake Flickr by Andrea_R

Light than can be seen by the eye is called the visible spectrum.  We can see light rays that have a wavelength from about 400 nanometers to 750 nanometers.  The visible spectrum is part of the electromagnetic spectrum which includes:

  • Gamma Radiation
  • X-ray Radiation
  • Ultraviolet Radiation
  • Visible Radiation
  • Infrared Radiation
  • Microwave Radiation
  • Radio Waves

If the wavelengths are slightly shorter than 400nm then they are considered ultraviolet rays and we cannot see them, if the wavelengths are slightly greater than 750nm then they are called infrared rays and we cannot see them.  We know X-rays, gamma rays, microwave rays and radio waves exist, but we cannot see them because they are too long or too short.

The Gemological Spectroscope

This is an instrument used to analyze the light from a gemstone.  The light through a spectroscope without a gemstone will show a rainbow that is the visible part of the electromagnetic spectrum. When the light goes through (transmitted) or reflects off of a gemstone a pattern of black lines or bands is seen through the spectroscope.  This pattern is called an absorption spectrum.

Diffraction Grating or Prism Spectroscope With Gems?

A prism spectroscope is constructed of a train of three or more prisms that are alternately made of flint and crown glass.  There is a collimating lens that focuses the light rays onto the first prism and assure that the light rays are parallel and produce a pure spectrum.  The dispersed rays produce the visible spectrum.

The diffraction grating spectroscope has fine lines and spaces (approximately 60,000 lines per centimeter) and the light is diffracted (bent) spreading the different wavelengths into the rainbow pattern of the visible spectrum.

The prism spectroscopes have an adjustable focus.  The amount of light entering the slit can also be adjusted.  The diffraction grating spectroscopes are smaller and do not have any moving parts.  If you are examining the spectrum produced by a prism spectroscope the blue area will be spread out and the red area is compressed.  The diffraction spectroscope distributes the wavelengths evenly.  When comparing the spectrum that you see to published spectrum, note which  type of spectroscope you are using and which type of spectrum is included in the published spectrum.

Fraunhofer Lines are a Solar Spectrum of Absorption Lines

Fraunhofer Spectrum  Lines

Fraunhofer Lines Flickr photo by yellowcloud

A spectroscope pointed at the sun will show a series of very fine lines caused by the elements in the sun.  The lines correspond to wavelengths that have been absorbed by a particular substance.  Absorption lines are formed when an electron makes a transition from a lower, to a higher discrete energy state, with a photon being absorbed in the process. The absorption lines or bands show as black lines or thicker bands against the coloured background.

Oxygen Emission Spectrum

Emission Spectrum Flickr by Image Editor

Above are the emission lines produced by oxygen. Emission lines are seen through the spectroscope as bright coloured lines and are formed when an electron makes a transition from a particular discrete energy level of an atom, to a lower energy level emitting a photon of a particular energy and wavelength.

The Spectrum of Gemstones

Apatite Absorption Spectrum

Apatite Absorption Spectrum by J McKercher

The black lines or thicker bands are the part of the visible spectrum that have been absorbed by the elements causing the colour in the gemstone.  Some elements can produce an emission  line.  Gemstones that produce a spectrum produce a unique pattern of lines and/or bands that allow the gemologist to identify the gemstones.  Not all gemstones will show an absorption spectrum and the direction in which that you observe the gem will determine what spectrum is seen.  The absorption spectrum above is for a gemstone called apatite.  It is showing a rare earth spectrum caused by the elements such as praseodymium and neodymium making up the rare earth elements.  This is the pattern for yellow apatite.

The spectroscope is a diagnostic instrument and does not require any toxic liquids and does not need batteries or electricity.  The smaller varieties fit in your pocket and can be used with a small flashlight.  This is one of the most useful instruments available to the gemmologist.

Your Input

(Join our poll or Scroll Down To The Bottom and leave a comment) – Emerald is winning the poll!

Visible Spectrum

Visible Spectrum Flickr by *Tom*

I have introduced different gemstones alphabetically.  I started with “A” (Ammolite) and it has taken me to Opal!  Leave a comment and let me know if you have a favourite gemstone that starts with the letter “P”.  Take the poll!  Can you name a reference to a gemstone in a song, movie or book?

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About Jen McKercher

Jennifer McKercher teaches gemmology at the Canadian Gemmological Association. A passion for gemstones drives Jen to learn as much as she can about the wonders of gemstones and how they enhance our lives.

Posted on December 16, 2011, in Gemmological Tools & Books and tagged , , , , , . Bookmark the permalink. Leave a comment.

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