Many gemologists have difficulty mastering the direct-vision spectroscope. This article eases the pain.
Gem testing laboratories make use of a variety of instruments, both high-brow and low. Of primary importance are the microscope, refractometer, ultraviolet light, specific gravity equipment, and the spectroscope. Among these, the direct-vision spectroscope is generally the most difficult to initially master. This fear factor leads students into a self-fulfilling cul-de-sac where lack of skill leads to lack of success and the eventual conclusion that the spectroscope is of little use. Nothing could be further from the truth. The spectroscope is one of the most powerful instruments in a gemologist’s quiver. But, like any weapon, you hit little if you depend on luck alone. You gotta practice.
The tremendous value of the spectroscope as an analytical tool lies in its ability to save time in gem testing, by providing initial identification of many gems, with the microscope being the only other tool required.
For example, let’s say you are given a dark blue transparent stone set in a closed back mounting. The construction of the mounting does not permit the refractometer to be used. With the microscope and fiber-optic illumination you locate a small well-formed transparent crystal in the gem, and using the fiber-optic light with the spectroscope you observe absorption lines in the blue region at 450, 460 and 470 nanometers (nm), a typical iron-caused visible light spectrum.
The conclusion is obvious: this gem is a natural sapphire, and the condition of the included crystal means the sapphire has not undergone high-temperature heat treatment. A complete identification was possible in this case with just two instruments. An added bonus in this instance was that no refractometer was needed, which saves the contact surface of the hemicylinder for when it is actually needed.
Other examples of the spectroscope’s utility are in the determination of dyed green jadeite from natural Cr-colored jadeite, and in the detection of irradiated yellow diamonds, where the instrument is of crucial value. There is really no other way for a gemologist to get this important information except through spectroscopic examination.
Whether you have a small pocket-sized diffraction-grating spectroscope without a built-in scale, or a table-top prism spectroscope with a built-in numerical scale, using a spectroscope is simply about pattern recognition. No matter which instrument you use, the positions of the absorption lines and bands will always be the same for any particular gem species that shows an absorption spectrum. As for example, iron lines will always be in the same areas of the blue region in an iron spectrum of a sapphire. The absorption pattern shown by a ruby will always be the same and in the same position in the visible light spectrum.
From almandine garnet to zircon, dozens of gems show diagnostic absorption patterns. The key to recognizing these patterns and associating them to a specific gem is practice. Whenever possible use your spectroscope. Put together an inexpensive set of spectral reference stones for practice, and refer to articles and books that contain drawings of spectra such as Richard Liddicoat’s Handbook of Gem Identification, or Basil Anderson and Alan Jobbins’ Gem Testing. The way to spectroscope proficiency is practice. Learning the proper use of the spectroscope is easier than you think.
Successful spectroscopy is largely a question of path length through the gem. Longer light paths allow more absorption, thus strengthening faint lines; shorter paths produce less absorption and so allow distinction of individual lines within areas of heavy absorption. Path length is determined by the following:
- Position of the gem: Stones must be carefully positioned so that only light passing through them reaches the spectroscope. Due to the arrangement of facets or inclusions, light may exit the stone in several different directions; the stone should be positioned so that the maximum amount of light passes out towards the spectroscope. Placing one’s hand around the stone in different positions allows one to determine exactly where the light is headed. Oval stones should be positioned so that the broad side faces the spectroscope. The stone’s position must also take into account color zoning. Absorption lines normally result from color, so the stone should be positioned so that light passes through deeply colored areas, to maximize absorption.
- Light source and its position: The light source itself is of tremendous importance. It must be intense, and focusable to a narrow spot. Fiber-optic illuminators of 150 watts or more work well for spectroscopy. A small angled light guide can then be held against the stone. The light may be positioned to allow transmission directly through the stone from below, or reflected from above. In most cases, the reflection method is superior, for it allows a longer path through the stone and, thus, more absorption. If too much absorption is seen, the path should be shortened by moving the light and/or stone to lessen the absorption. For example, shortening the path in Fe-rich Thai/Cambodian rubies may allow one to pick out the iron lines at 451.5, 460, and 470 nm, in addition to the Cr spectrum. Lengthening the path intensifies faint lines, which allows one to see the 451.5 nm complex in heat-treated Sri Lankan sapphires, where it is normally weak.
Once one understands the implications of path length, it is simply a question of positioning the instrument in the proper spot to catch the light. Always make sure the spectroscope is firmly anchored, either with the supplied holder on table models, or with modeling clay or plasticine.
For many years, the king of gemological spectroscopes was the Beck prism model, a five-prism jewel that has assumed almost legendary status amongst the gemological cognoscenti. Sadly, it has not been manufactured for over since the 1980s and units today are coveted.
But there is an alternative. The mid-1980s saw the rise of inexpensive fixed slit diffraction-grating models from OPL, bringing top-quality spectroscopy within the reach of the commoner.
We use the large OPL "teaching" spectroscopes with the stand. They are affordable enough that every gemologist can have one, and when coupled with the same fiber-optic light source used for the microscope, gives gemologists a powerful tool..
Diffraction grating units have several advantages. First, the fixed slit means one less adjustment to fret about. Even more important, diffraction-grating spectra do not suffer the bend-me-crush-me extremism common to prism models, which simultaneously pull the violet and push the red. The result is a balanced display of colors. Not only does this make faint lines in the blue and violet easier to resolve, but tightly packed absorption striae at the red end spread out for greater visibility. We can almost picture Buddha nodding in agreement. Balance, the middle road. Oh, yeah!
Visible Spectra of Corundum
|Variety||Spectra description (from 400 to 700 nm)a|
|Ruby (including pink)||
Vanadium spectrum (common in synthetic corundum; rare in natural corundum)
a. All wavelengths are approximate only.
b. Pleochroism can have a slight effect on the absorption spectrum. By rotating a polaroid plate over the spectroscope eyepiece, the spectra of both the ordinary and extraordinary rays can be viewed independent of one another.
Cos Altobelli, the award-winning AGS gemologist, appraiser and jeweler, summed up the importance of the spectroscope this way: “For most gemologists and jewelers, the spectroscope is like a tuxedo. While you don’t need it often, when you do, there is no substitute.”
So dust off the tuxes and ball gowns. And, while you’re at it, make sure your spectroscope is working, too, because you never know when it will be needed.
About the authors
John Koivula is the author of the magnificent Photoatlas of Inclusions in Gemstones, Vols. 1–3, along with several other books and over 800 articles. He is currently Chief Research Gemologist at the Gemological Institute of America and is the world's foremost gem photomicrographer. John was also the scientific advisor to the famous MacGyver television series. Many of his books and enlargements of his images are available through microWorldofGems.com.
Richard W. Hughes is the author of the classic Ruby & Sapphire and over 170 articles on various aspects of gemology. Many of his writings can be found at www.lotusgemology.com and www.ruby-sapphire.com. His latest book is Ruby & Sapphire: A Collector's Guide (2014).
This article was originally penned as one of our regular Elemental columns for the American Gem Society's Spectra magazine in 2005–2006. It has been updated to include information from Hughes' 1997 book, Ruby & Sapphire.