Gemmology Canada, Number 41, April 1995 - Special Issue
by Donald G. Coughlin
The method of Visual Gemmology (V.G.) is the human equivalent of a gemmological refractometer used for estimating the refractive index (RI) of gemstones. Under normal circumstances, V.G. may never replace the refractometer (See table of comparisons below) - but under abnormal circumstances it is undoubtedly the next best thing.
Dense prism The eye ball Lens The pupil Produces 1 or 2 shadow edges Produces 1 or 2 lines Requires RI liquid Yes No Requires light source? Yes Yes Determine pleochroism? No Yes Anomalous D.R.? Yes Yes Indicates D.R.? Yes Yes RI Scale? Yes No DR above RI 1.81? No Yes Optic Character? Yes No Optic Sign Yes No Diagnostic? Yes No Test set stones? Few Most
When you look through the table facet of a cut gemstone that is nearly touching the eye, and face a single light source (light bulb, cigarette lighter, etc.) you see a geometrical pattern. Each facet produces either a single or double light source image, and in turn, each light source image produces A SINGLE LINE (Isotropic), OR DOUBLE LINES (anisotropic), leading from one light image to the next (like a spider's web). The separation between these lines is the measurement of the D.R. of the stone being tested - and corresponds to the RI shadows shown by the refractometer.
If you know the Double Refraction of a stone as seen through the refractometer then you already know Visual Gemmology! The major failing of V.G. is the lack of a refractive index scale. You will note that the higher the RI or Dispersion of a stone, the greater the angle at which you must hold the stone in relation to the light source. As a result, when you have difficulty in locating the images, you know the stone has a high RI - sufficient in many cases to differentiate between two different stones of similar colour.
Example - brown topaz v brown zircon:
Topaz (mean RI 1.63, DR 0.008) - You must move the stone slightly (about 30 degrees) to one side of the light source to observe a portion of the V.G. pattern, and the twin lines are separated by about 1/8".
Zircon (mean RI 1.95, DR 0.059) - You must move this stone close to 80 degrees away from the light source. Note that the lines are now separated by about 7/8".
The above would be diagnostic for zircon - while following instructions for use of a refractometer one is told that if you receive a negative reading it is safe to assume it is zircon!
It is sad how little attention is given to V.G., and I do not understand why! In a place like Sri Lanka it is invaluable, and can be performed anywhere, and at any time - without expensive equipment. V.G. separates glasses from natural stones, synthetic spinel imitating topaz and aquamarine. In other cases, V.G. can at least indicate whether a stone could be the stone it is purported to be. In this respect V.G. should be considered at least as important as the Chelsea filter or dichroscope. In fact I use V.G. on every cut gemstone before turning to other instruments.
Most of the time I do not have testing equipment with me when I need them, and carry only my trusty loupe and a penlight. This is when V.G. pays off! V.G. sometimes can be confirmatory, e.g. heavily included Sri Lankan hessonite garnet, but I repeat: It serves only to determine what a stone could be - or could not be.
I would recommend that if you seriously intend to learn V.G., you do like I did, experiment and experiment! I had no notes on the subject, and had never even heard of such a thing as V.G., and discovered it quite by accident when looking for pleochroism by eyeballing a gem under an incandescent light bulb at home. I noticed the double lines, and wondered if an isotropic gemstone would be the same. I then took a spinel to compare and voila! From that point I tested hundreds of stones, until it became second to nature to determine approximate DR at a glance. You can do the same, and I am convinced VISUAL GEMMOLOGY MUST BE LEARNED ONLY BY EXPERIENCE AND NOT FROM THESE, OR ANY OTHER NOTES.
Possbily this is the reason why V.G. has not acquired proper acceptance - everyone wants a text book!
Here is my alternative to a text book which may help you get started:
Eye-glasses: Remove them, they are unnecessary and inhibit the use of V.G.
Incandescent light source: As your light source, use either a ceiling lamp fixture, or table lamp (shade removed), with a clear light bulb installed. In a pinch I have used my cigarette lighter, and even a match.
Note: The distance from the light source to your eye is unimportant, whether it is several feet in the case of a ceiling lamp, a few feet for table lamp, or a few inches for penlight.
It is important to accustom yourself to working in a well lit room despite the fact that V.G. is easier performed in the dark. The reason for this is because it can be quite annoying to have to continually switch on/off the light. And, even more important - you will be using V.G. in any situation, such as in jewellery shops or Sri Lankan "Pola" gemstone street markets. (I doubt very much if jewellery store staff would appreciate you turning off their lights, particularly if there are diamonds lying about).
Gemstones: A beginner should start with stones as large as possible, e.g. 1.5 carats or more. It is important to note that when you have had sufficient practice, you will have no problem testing stones of .5 carat or less, and STONES SET IN jewellery!
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Select a faceted spinel, garnet, or glass. Hold the stone with the table facet very close to your eye - even touching your eye lashes with it. To avoid eyestrain, try keeping both eyes open, using only your "seeing" eye as you should do when using your 10x lens. With practice it becomes quite easy to master. If the stone is a large one, use your fingers to hold it, otherwise use tweezers (Editor: Be very careful here as the stone may pop out and end up in your eye!).
Again, I must emphasize that the stone must be kept clean - finger smudges on the stone blur the images. Point the stone directly in front of the light. Don't be discouraged if you find it difficult to locate the light images. Looking through the stone, move it slowly away from the light until the images appear. Now, take a moment to admire the colourful kaleidoscope of images that confront you. Note carefully the light source images (one for each facet), and even more important - note the lines of light leading from one light source to another. It forms a beautiful "spider's web". The fact there is only a single light-source image, with a single line leading from light image to light image, indicates the stone is isotropic.
BEWARE: Spinel or garnet are notorious for anomalous double refraction (ADR). If you see an erratic double line leading from one light-image to another, follow around the whole pattern and if most of the pattern consists of single lines (or single light source images) this as ADR. With the Visual Analyser Device (VAD) - an instrument described below - this is not a problem.
A hessonite garnet is the easiest of all the popular gemstones to identify with V.G.. Try it! You will only see a very hazy images of the light source, with the whole visible background appearing as hessonite-coloured snow (a beautiful abstract painting!) Certainly no other "look-alike" stone can be confused with hessonite, using V.G.
Anisotropic (Doubly Refractive) Gemstones
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V.G., like all other gem testing methods, requires practice (lots of it), and it helps tremendously if you memorize a simple table of DR's. Here is a simple method to do it: Learn only the actual numerics of DR's, dropping the unnecessary zero(s) and decimal point (e.g. DR for peridot is 0.036, so we note it as 36, etc.) Memorizing DR's will also aid gemology students for use of the refractometer as well, but remember to put the dropped decimals back in place at examination time! A table of helpful DR stones, in ascending order, for use in Visual Gemmology follows:
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Beryl 6 (mean)
Zircon (high) 59
Let's look at the quartz gemstone. What do you see? Double light-source images, with twin parallel lines leading from each light image to another. These twin images are an indication that stone is anisotropic, and give you the estimated DR. (But watch out for the anomalous double refraction mentioned under "Isotropic" above!)
YOU DO NOT HAVE TO LOOK AT THE WHOLE PATTERN - ONLY A SMALL SECTION OF IT.
Make a mental note of the distance between one set of twin parallel lines. In quartz with a DR of 9 they will appear approximately 1/4 inch apart. If you really want to master V.G., then practice making a mental "photographic imprint" of the pattern, especially the distance between twin lines. Just flash it in your minds eye, look away and you will be able to reproduce it fairly accurately - mentally.
Now, select a stone with a larger DR, such as tourmaline with its DR double that of quartz - 18. The twin lines will now be 1/2 inch apart, or at least double whatever estimate you made for quartz.
After studying those two stones, select a peridot with its DR (36), double that of tourmaline (or four times that of the quartz. Now the distance between the twin lines are proportionately larger than previously noted readings. If you do not have peridot, try a zircon (DR 59). With zircon, however, you will have difficulty at first in locating the images due to high RI. By turning the stone in any single direction parallel with the light source, or moving your eye up and down (or side to side) you will capture the images. Now you should have the DR's of 3 or 4 stones imprinted in your mind's eye, and these will be a guide to learning other gemstones with their respective DR.
Note: When you experience difficulty in finding the line(s) or light source image(s), you must turn the stone in every direction until you find them - and this takes lots of practice. Sometimes it is necessary to view through the girdle, etc. and in these cases you have to watch very carefully to glimpse a small portion of the image. In such cases there may be no lines, so you must concentrate on observing only the light source image(s).
I make V.G. easy by using, what I call:
The Visual Analyser Device ("VAD") is my own, home-invented, instrument which I find absolutely necessary to effectively practice the art of V.G.. In fact it should be a basic instrument in every gem tester's arsenal.
The construction is almost too simple to be believed, but the results are excellent. After several "flops", I made two working models: One is a tabletop model, and the other is a hand-held portable. The portable model is essential for testing gemstones when no other instruments are on hand, e.g. out in the field.
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HARDWARE SHOP: - One large size lantern with ordinary single light bulb (Table Model). I use an inexpensive emergency lantern, rechargeable, with swivel top lamp and a normal light bulb. The base has two fluorescent daylight lamps (useful to test colour-change stones). - Black electrical tape. - Epoxy Cement ("Crazy Glue" or equivalent is fine)
PHOTOGRAPHIC SHOP: - Step-up adaptor ring 49-55mm - 49mm Polarizing lens, good quality (marked PL on the wider rim). You will notice that the PL consists of two independent rings, one with the milled edge is normally the polarizer and the the other is the accessory ring for attaching to a camera. TEST BEFORE BUYING! Look through the lens at any fairly dark surface that reflects light (I used my suntanned arm!) Place the PL in front of - and close to - your eye. While looking through the PL, rotate it 90 degrees and you should see the polarizing effect where the surface tone changes from dark to light. Try several PL's and select the best one - but be careful not to purchase one of the "special effects" polarizers. This type of PL is treated so that half of it gives a blue or other shade which makes it totally unsuitable for determining pleochroism.
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Step 1: Using the black electrical tape, cut a tiny hole (about 2-3mm) in the first strip. Align the hole directly over the light bulb assembly, and stick the tape down well. (Switch on the lamp while doing that, as it is critical that the hole is centred.)
Step 2: Tape the rest of the lamp shade (glass or plastic) so that no light is seen except through the hole.
Step 3: Cement the adaptor ring (on the 55mm side) directly over the taped lamp head, and let it dry.
Step 4: Screw the 49mm polarizer on and voila! You now have a VAD with removable filter.
NOTE: If the polarizer does not provide the required blinking lines or light images, then REVERSE the polarizer (both sides have 49mm threads). I guarantee satisfactory results!
When you become more expert at V.G., you can experiment by screwing 49mm colour filters on to the polarizer (stacking if desired), e.g. filters may be red and/or blue. I have found that with a red filter some stones show a clearer DR pattern. In fact using the red filter on the VAD, I can see the small DR of 0.003 in taaffeite - but it does take experience to spot this.
HOW TO USE THE VAD: With a doubly refractive stone as close to the eye as humanly possible, look through the table facet, and ROTATE THE POLARIZER CONTINUOUSLY as you search out the double lines. With each 45 degree turn you will see two rays blink on and off - first one - and then the other. This is diagnostic for double refraction, and you do not have to worry about anomalous DR as you would if practising V.G. without the VAD. Also this eliminates the many difficult instances where lines are very close together (sapphire), far apart (high zircon), or even overlapping such as occurs under the long fluorescent light tubes.
I have assembled a small Portable VAD using a rechargeable flashlight, and the head is perfect for the 49mm polariser. I cemented the polariser directly onto the flashlight rather than employing the adaptor ring. Let your imagination go wild!
ADVANTAGES OF VISUAL GEMMOLOGY
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a) V.G. (especially using the VAD) is useful in separating the "look-alikes" such as: -brown tourmaline (DR 0.018, from sinhalite (0.038) -sinhalite (DR 0.038), from zircon (0.059) -aquamarine (DR 0.006), (which requires very close scrutiny because of its small DR,) from blue topaz (0.010) - chrysoberyl (DR 0.009) from sinhalite (0.038) and zircon (0.059) - yellow scapolite (DR 0.020) from yellow quartz (0.009). This example is excellent since scapolite and quartz have similar RI, and if using the refrac
b) Anyone can learn V.G. easily, and at little or no expense.
c) Useful for testing stones set in jewellery, e.g. bracelets, rings, pendants.
d) Useful for assessing pleochroism.
e) Most important: V.G. is often the only gemstone test one can perform out in the field, that gives two important factors: (1) Double or single refraction, and (2) if double, it shows the approximate DR measurement.
ON THE NEGATIVE SIDE
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a) V.G. is often too difficult to use on many stones if you do not use a VAD. A good example is a Sri Lankan blue sapphire which is dark, has a small DR of 0.006 and high RI of 1.76. I rarely use V.G. for this type of stone since it requires proper testing anyway.
b) V.G. should be used strictly as a supplement to other testing methods, e.g. simply as a guide - RARELY as a final diagnosis!
Look through the stone in several directions, just like you would through a dichroscope. It may not as easy to determine pleochroism with V.G. as it is with the dichroscope, however it is still useful. View the stone exactly as you would when searching for double refraction, but recommend you use natural daylight as the light source. Concentrate solely on colour as you rotate the stone, and it is impressive to see the whole scene change colours with pleochroic gemstones. Trichroic stones necessitate remembering all three colours, the same way as when using the polariscope. Important note: In determining pleochroism, V.G. is excellent for both cut and rough stones.
To sum up, V.G. leaves us a whole new and exciting world to be discovered. It is my sincere hope that much more attention will be paid to this particular field of gemstone identification. Armed with the VAD, or improved versions of it, an enterprising and questioning mind may even figure a way to determine the optic sign! What about a printed RI scale one holds out at arm's length in the vicinity of a portion of the double lines? Hurry - or I may beat you to it. Even now I have a good idea of exactly what the construction of a professional VAD would require.
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1. Bauer,M. (1896) Edelsteinkunde, Precious Stones (English translation, 1903), 49ff
2. Crowningshield, R & Ellison, J.G. (1951) Determination of optical properties without instruments. Gems & Gemology 7, 120-124
3. Hanneman, W. (1980) Educating the eyeball - The Hodgkinson Method, Lapidary Journal 34, 1498-1519
4. Hanneman, W. (1982) Understanding the Hodgkinson Method, J. Gemmology 18, 222-225
5. Hodgkinson, A. (1989) Visual Optics, Australian Gemmologist 17, 137-138
6. Hodgkinson, A. (1970) Visual Optics, J. Gemmology 17, 301-309
7. Mitchell, K. (1983) The Oughton spectra, J. Gemmology 18, 382-384
This article was first published in Gemmology Canada, Number 41, April 1995.
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