About Spectroscopes

 

Gem Spectroscopes & Spectrometers

There are several ways of identifying colored gemstones and diamonds. One of the most accurate mechanisms and one that can be employed on rough or polished stones that are unmounted or mounted in jewelry, is spectral analysis. Every mineral and gem that transmits, reflects or emits light has a unique signature due to its chemical composition and crystal structure. When ‘white’ light is passed through or reflected from an object, certain colors are absorbed and others are transmitted or reflected. This is what gives any viewed object its perceived color.


SELECTIVE ABSORPTION. When white light passes through a substance it is selectively absorbed at different wavelengths and the resultant light that passes through provides the color of the object. White light can be broken down into its component colors: red, orange, yellow, green, blue, indigo and violet and each color travels at a different velocity and has a different range of wavelengths that define it. Colors other than the spectral seven are cause by the mixture of various wavelengths of light. Visible light ranges from about 390 nanometers to about 750 nanometers in wavelength, and the wavelengths range immediately below violet is the ultraviolet and those beyond red, infrared. While humans have no visual acuity in the infrared and ultraviolet, spectrophotometers are instruments that can record absorption or transmission in those wavelengths in addition to visible light, and our Challenger Gemological Spectrometer can “see” spectra in the near infrared (750-1000 nm).


Selective absorption in gems is caused by crystal chemistry, the combination of a gem’s chemical composition and its crystal structure. When one looks through a spectroscope one will often find one or more diagnostic absorption lines or absorption bands that characterize the material.


ABSORPTION LINES are dark, sharp vertical lines that are typically thin and well defined.


ABSORPTION BANDS are also vertical but they cover a range of wavelengths and are broader than absorption lines and generally they are not as well defined and may appear fuzzy.


PRISM & DIFFRACTION GRATING SCOPES.  Prism spectroscopes use dispersion to break light into its component colors and all early spectroscopes were prism units and these also tend to be the most common types used in gemology. Diffraction grating scopes use the principal of diffraction where light enters through a slit and impacts a thin film of diffraction grating material. There are not many diffraction grating spectroscopes used in gemology but a high resolution diffraction grating provides superior results and is used often in astronomy and also in the two gemological spectrometers made here by Imperial Gem instruments. As a practical matter in a diffraction grating the spectrum is more linear where with a prism scope the spectra appears expanded in the blue and compressed in the red.





Many of the chemical elements that make up a gemstone absorb light at very specific wavelengths. Therefore, each material has a special signature that is as unique and identifiable as a human fingerprint. This makes the spectroscope extremely well suited for use in identifying colored gemstones and diamonds.


TRANSITION METAL CATIONS are the elements that typically cause the absorption lines and bands responsible for the distinctive color of gems. The most common transition element to cause color is iron. Other transition elements include chromium, vanadium, cobalt, titanium, nickel, manganese and copper. Those gems that have the transition elements as part of their critical and distinctive general chemical composition are said to be IDIOCHROMATIC. Gems that have trace amounts of transition elements as impurities in the crystal structure are said to be ALLOCHROMATIC.


The most common gemstones that have distinct spectra are:


Ruby

Sapphire

Emerald

Diamond

Spinel

Garnet

Jadeite

Peridot

Chrysoberyl

Zircon


In addition there are literally scores of additional gem species that have distinctive spectra. We’ve compiled a list available at the end of this section.


DIFFICULTIES WITH HAND SPECTROSCOPES. The spectroscope is one of the more difficult gemological instruments to use because there are a wide variety of techniques that need to be mastered in order to use a spectroscope effectively. This is a major reason we developed our instruments - to improve ease of use and accuracy. To do a spectral analysis with a conventional hand spectroscope properly is also quite time consuming as an important part of the process is to do the viewing in a dark room after one’s eyes have become accustomed to the darkness and have maximum sensitivity. A light shield should also be erected to keep the light source from dazzling the eyes and ruining the night vision. Of course you may also need to be able to see printed reference spectra and balancing that with keeping the eyes acclimated to the dark can be a challenge.


As we age, the vitreous humor of the eye becomes yellow and reduces our visual acuity in the blue portion of the spectrum and thus absorption lines in the blue become increasingly difficult to see with age. This makes acclimating to darkness even more important with a conventional scope - it’s not an issue with our equipment.


Many spectroscopes do not have scales of any type so it is not possible to refer to the wavelength location of the absorption line. As a result, gemology students are taught to recognize patterns. Of course the patterns are different if viewed with a prism spectroscope or a diffraction grating spectroscope so that, too, causes problems.


An anachronism and a huge disservice being perpetrated on gemology students is that the concept of pattern recognition is superior to actually recording the location of each spectral line against an internal or external wavelength scale. This might have been fine when only prism scopes were available and wavelength scales were rare, but now the pattern recognition technique as sole form of practice is in many cases inaccurate and flawed. Indeed, everywhere in the gemological literature the description of the lines in a spectrum include the location in nanometers or Ångstrom units.


COMPETITIVE SUPERIORITY. Imperial Gem instruments recently discontinued the Challenger Gemological Spectrometer but now builds the new MDM Direct Reading Digital Gem Spectroscope that matches the Challenger’s capability but is half the price ($1600). These units, the only ones of their type in the world, use diffraction gratings and B&W video cameras and monitors (B&W being much better at discerning the dark absorption bands than color equipment) with a separate digital LCD readout accurate to 1 nm. You can measure the wavelength of each absorption band with excellent precision and accuracy and you can do it very quickly without light acclimation. You can get readings in the near infrared, to 1000 nm, as well. With these units you can routinely see absorption lines you would have no hope of detecting with a conventional spectroscope and you can make identifications/separations that you could not make otherwise.


If you work in an environment where you regularly use a spectroscope for gemological applications you will be amazed at just how fast and accurate it is to use one of our instruments. It dramatically increases your productivity, capability and, perhaps most importantly, your confidence.