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CORRECT METHODS AND APPROACH IN GEM TESTING

Spectroscopy is one of the most powerful and broadranging methods of analysis, utilizing the visible portionof the electromagnetic spectrum. In definition, ‘spec-

troscopy is the science of analyzing the interaction of lightwith a material, either visually or mathematically via graphicalanalysis.’

Most materials will show spectral results, but they will bewithin different areas of visibility. Some require more special-ized equipment or conditions in order to be seen. In gemspectroscopy, we focus on areas of the electromagneticspectrum such as visible, infrared, UV and X-ray. In standardgemology, we use a handheld spectroscope analyzing thevisible range of the absorption spectra which is 400–700nm(nanometer nm=1 millionth of a millimeter). Figure 1.

Used with the right technique and confidence, it is one of themost useful tools a gemologist can possess in regards to ef-fective results providing both portability and versatility.

HOW DOES THIS ALL WORK?The perceived color of most objects is achieved in their abilityto absorb certain wavelengths or colors of light passingthrough them or reflected from their surface. The suppres-sion of parts of the spectrum in light is known as selectiveabsorption–you can think of these as a barcode or DNA pat-tern for each variety of gems. Figure 2.

We observe, measure and interpret the patterns and widths ofthese dark absorption bands and lines emanating from a gem-stone through a spectroscope. As gemologists, we need to beable to observe the patterns, positions and widths of these ab-sorption bands and lines to reach a possible conclusion to agems identity, or even be able to measure the position of thoseabsorptions more accurately using a numerical scale.

CAUSES OF ABSORPTIONSCertain coloring elements may be present within a gem-stone. These are called transition elements and help to giverise to absorption of various wavelengths of colors. Thereare 8 main transition elements that can give rise to variouscolors. Figure 3.

These transition elements absorb some of the wavelengthsmaking up white light as it enters the gemstone. The re-

HANDHELD SPECTROSCOPYClaire Mitchell, FGA, DGA

The spectroscope is a very useful gemological instrument that is underutilized by gemologists worldwide.Learning to master this tool will greatly help in practical gem testing and identification.

FEATURE ARTICLE

FIGURE 2.The absorption bands and lines seen in almandine garnet ©JohnHarris: Gem-A Diploma in Gemmology, 2008.

FIGURE 1. ©Gem-A, 2008.

FIGURE 3.Metallic coloring elements in gemstones. ©Gem-A Foundationin Gemmology, 2008.

SPECTROSCOPY

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maining wavelengths (colors) are transmitted out of thestone in a combined form and are viewed by your brain asa single color. So, for example, if all of the wavelengths areabsorbed except in the red region of the spectrum, thegemstone appears red.

WHAT ARE THE CHALLENGES?Unfortunately, many gemologists find this piece of equip-ment challenging at best. Initial unclear results can lead tofrustration and disappointment, but it is usually fault in tech-nique rather than the gem that causes this. Having said that,not all stones will provide a clear spectrum and some maynot show one at all.

It is my hope that the following can inspire you to try to revisitor refine your use of this great gem tool and achieve and in-terpret results with confidence. As in most cases with gemol-ogy, practice and patience are key.

Using a handheld spectroscope, we can test transparent,translucent, faceted, rough, polished, unpolished, loose, or

mounted items, as long as we can get light through thestone. Also, your stone should, with the exception of a fewgems, have color.

TYPES OF SPECTROSCOPES There are various types of spectroscopes available on themarket today. Some come with holders or stands for thegemstone. Some have the ability to focus or have an ad-justable aperture, which allows the light intensity enteringinto the spectroscope to be altered. But they all use one oftwo methods to produce a visible spectrum as a band of col-ors: Prism or diffraction grating. Figure 4.

While the prism spectroscope is ideal for the brightness ofthe spectra it provides, it does not provide a linear spec-trum. It compresses the red end and spreads out towardsthe violet. The diffraction grating spectroscope provides alinear spectrum. There are advantages in the use of both.But if you are just starting testing I would recommend thediffraction type. Figure 5.

IMPORTANCE OF LIGHTINGUsing the correct type and method of lighting is of vital im-portance for successful results. Most poor readings are dueto incorrect lighting technique. First, check the light sourceby looking at it directly with your spectroscope. This way, youcan establish that your light source does not have its own ab-sorption or emission spectrum, which will interfere with theobservations you are carrying out on your gem. This couldlead to misleading or confusing results. Ensure that you aretesting in a dark environment.

A good strong light source is essential. You can use a flash-light but make sure your batteries are fresh and strong. Withdeeper colored stones, or more translucent stones such asjadeite, a fiber optic light can be used to produce the best ef-fect. You can use either reflected or transmitted light to illu-minate your gem. Figure 6.

GET YOUR GEMSTONES GLOWING! Correct illumination technique is critical. This is where mostpeople start to experience problems. The maximum amountof light must pass through the stone; the stone must reallyglow. Figure 7.

Additional light not going through the stone can overpowerthe absorbed light, meaning that little or no absorption patternis seen. Not enough light through the stone limits us seeing

FIGURE 4. Lower Image: Cross section of a prism spectroscope. Upperimage: Cross section of diffraction grating spectroscope. ©Gem-A, 2008.

FIGURE 6. Left image: Using a spectroscope with reflected light. Note thatthe stone should be placed on a black background to get the best resultswith this method. Right image: Using a spectrum with transmitted light.Note to take care when using a bright light not to blind yourself, or over-power the spectrum. ©Gem-A Diploma Foundation in Gemmology, 2008.

FIGURE 5. Above: The linear spectrum of a diffraction grating spectroscope.Below: A spectrum from a prism spectroscope with the colors compressedtowards the red end. ©Gem-A Foundation in Gemmology, 2008.

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any absorption pattern. In pale or dark stones, if the stone isdeep in color and translucent, illuminate through the thinnestpart. If you have color zoning make sure the light is passingthrough the deepest colored part. Heavily included stones orincreased translucency may hinder. This is where a reallystrong light source comes in handy.

If using the transmitted method, place some sticky tack atthe end of the light, which helps to support the stone, as wellas helping to avoid extraneous light passing around the sidesof the gem. If using the reflected method, make sure yourgem is laying on a dark, non-reflecting surface, ideally tablefacet downwards. Some spectroscopes come with holdersor even stands on which you can place the stone and securethe spectroscope for optimum viewing. Alternatively, a mi-croscope could be used with the spectroscope placed in theeyepiece, low magnification, and bright field light source withiris lens. Make sure you do not look directly at the bright fieldwith the gem iris not in place.

It’s important to remember that spectra given in reference areusually text-book, bright clear and defined. For those of us whogem test on a regular basis we know that unfortunately MotherNature does not always bless us with text-book results.

WHAT ARE WE LOOKING OUT FOR? Let’s take a look at two gems which are colored bycobalt. A typical cobalt spectrum consists of three broad

absorption bands in the red-orange, orange-yellow andgreen. Above you can see illustrated two different gems,colored by cobalt with very similar spectra. Let’s take acloser look at the position and widths of their bands. Theslight difference in their thickness is the key to their iden-tity. Please note: natural cobalt colored blue spinel canbe found and although rare, its spectrum could be con-fused with synthetic, therefore, further testing is recom-mended. Figure 8.

In summary, the spectroscope is portable and can be diag-nostic as to a gem’s identity, but as any good gemologistknows, observation must still be carried out initially beforeany additional instrumentation is used.

This article has provided the basics for using the handheldspectroscope and has hopefully inspired and encouragedreaders to revisit or persevere with the spectroscope. At-tend practical workshops or lab classes on the subject tolearn more and refine and improve technique. u

Claire Mitchell and Eric Fritz of the Gem-A will be presenting a hands-on workshop on Mastering Your Spectroscope following the World of Gems Conference on September 25, 2017 in Rosemont, Illinois. Sign up at www.worldofgemsconference.com.

SPECTROSCOPY

FIGURE 7. Left image: Light leaks from around the stone producing poor re-sults. Right image: For best results, light passes through the stone creatinga glow effect. ©Claire Mitchell Gem-A 2016. FIGURE 8. ©John Harris: Gem-A Foundation in Gemmology, 2008.