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INDEX
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Identifying Faceted Stones Without Costly Gear
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Basic gemmology for facetors does not necessarily mean spending hundreds of dollars on equipment and does not require the facetor to acquire the language of a qualified jeweller. Indeed, one system, for faceted stones only, introduced by a canny Scotsman,
requires equipment as simple as a pocket spotlight, a strip of black plastic drilled with a small hole, a bit of Blutak and if you want to really spend up big, a polariser, such as one from your camera or one eyepiece from a broken pair of polaroid
sunglasses!
The beauty of this system is that you can use it on mounted stones provided the back is open and you can see your light source through the table. This basic equipment will permit you to make informed estimates at the refractive index of any faceted stone
you examine, whether it is doubly refractive or not and a close approximation of its dispersion.
The system, "Visual Optics", was created over 25 years ago by Alan Hodgkinson, of Scotland, who discovered its mechanics whilst working as a taxi driver in London when he was studying gemmology. Waiting for a fare one very wet night, Alan pulled some
faceted stones from his pocket and began looking at them using street lights as a light source.
Pointing the culet towards the light and placing the polished table as close as possible to his eye, he discovered each different gem material had its own spectra ( that is, a break-up of the light as it was collected by facets on the culet.
Alan Hodgkinson realised it was possible to approximate the refractive index by the positioning of the spectra as you look towards the light source Low RI spectra appear close to the centre of the stone. High RI, say, of zircon, (1.96) which is beyond
the scope of most refractometers, has a spectra very close to the outer perimeter of the stone. Diamond can be easily identified because its RI (2.41) is so high, its spectrum cannot be seen at all through the table. The difference between cubic zirconia
and diamond is easy to pick because by using this system you can see the spectrum of CZ close to the girdle area of the stone, but only the secondary spectrum of diamond which closely resemble bright coloured narrow darts all over a near-black
background.
To understand the theory of "Visual Optics" one needs to read the book on the subject, "Visual Optics - Diamond and Gem Identification Without Instruments" published by Gemworld International Inc., of Northbrook, Illinois 60062, USA.
Most facetors understand the way light bends as it passes through the faceted gemstone from the table, and if angles are correct, is reflected back to the eye. The light rays bend very little in the case of quartz (RI 1.54) but bend very considerably for
corundum (1.76) and bend almost at right angles for the higher RI materials such as cubic zirconia and diamond.
Using this system, you are, in fact, reversing the light path inasmuch as it passes from your light source, through the culet of the stone to the table and to your eye. The light rays are still bent, but in the opposite direction.
Let's stop trying to visualise the process and do it! Find a strip of black plastic, say 8 cm long by about 3 cm wide. and as thin as possible. Close to one end drill a 4 mm hole and if you have some large faceted stones, drill a hole at the opposite end
of 5 to 7 mm diameter. Clean a faceted topaz and place the table face down over one of the drillholes and hold it in place with two blobs of BluTak. If you have a very large stone you can hold it to your eyeball with your fingers. (Use of tweezers and
forceps is not recommended for safety reasons).
Now with the room darkened, point the culet at your pocket spotlight at a distance of say, two to three metres. If you wear spectacles, remove them. Bring the table as close to your eyeball as possible. Look directly at the light source and after a few
seconds you will see something quite remarkable. For every main facet on your stone, you will see a spectrum with the red end pointing towards the tip of the culet.
As you hold the stone still and move your line of sight around the hole in your plastic strip you will quickly locate the other spectra. Take particular note of the position of the spectrum between the girdle of the stone and its culet. That positioning
will apply to all stones of similar cut in the same refractive index range of 1.62.
If you have a faceted peridot, affix it to your viewing strip and repeat the process. You will notice two things immediately. The spectra are much brighter and there are two of them tail-to-tail to each main facet on the culet. This tells us peridot is
strongly double refractive and the span of colours in the spectrum confirms that it has high dispersion.
The full spectrum runs from red (closest to culet) to violet (closest to the girdle) and the span of colours from the red end is the measure of dispersion in the gemstone. Again notice the positioning of the double ring of spectra in relationship to the
culet tip and the girdle. All similarly faceted stones in the same RI of 1.65 will show the same pattern. You will notice that low RI stones are almost clear as you look at the light source and as the RI increases the centre of the stone darkens until it
is nearly black in the case of CZ and diamond. While the colour density of the gem material will slightly alter this appearance, the positioning of spectra for all stones of the same RI will be constant.
In doubly refractive stones you will be able to tell without a spectroscope the degree of birefringence. There is that terrible word which simply means the spectrum is partly or totally repeated. The clue here is to look at the red sector. In peridot you
will see the red sectors are quite separate as they point towards the culet. With medium double refraction (birefringence) the spectra overlap and you see red reappearing part-way down the length of the spectra. In stones of low double refraction
(birefringence) the second red can be very close to the first, or even be a lighter shade of red or pink. This is a very important part of identifying gemstones of the same colour; ie garnet, ruby and red CZ for example.
With stones which are strongly double refractive and which have two directions of colour (dichroism) ( tourmaline is a good example ( it is wise to check as many spectra as you can find. To confirm the feature, place polariser between the stone and your
light source. Rotate the polariser between 90 and 180 degrees as you watch the spectra and if it is a truly double refractive stone, each of the spectra will blink on and off as you turn the polariser. You will notice after some practice that various
stones have similar or very dissimilar spectra patterns and this is due to the absorption of white light by component minerals in the gem. This is clearly shown with ruby (both natural and most synthetics) in that the spectra is simply red and blue with a
hint of green and with no other colours to be seen. Red beryl, for example, while having similar birefringence and dichroism, would show red and violet.
"Visual Optics" will permit the facetor, after some practice to readily pick diamond from CZ, topaz, white zircon white sapphire; peridot from green quartz and green tourmaline and blue corundum from blue topaz, iolite and so on.
This, with a good 10X loupe, a knowledge of inclusions, cleavage habits and other observations will give the facetor the ability to identify most gemstones. It is however, advisable to state your opinions as such and recommend consultation of a qualified
gemmologist if identification is of paramount importance.
Selection of the light source is reasonably important.. Alan Hodgkinson and his American associate, William Hanneman, prefer a torch with a slitted black cover. For obvious reasons fluorescent tubes are not used with this system. Alan has the used the
moon as a light source when testing zircons and he has used light reflected from a chrome pen clip to test a range of faceted gems!.
The Aussies now use a 12-volt 5 watt tungsten festoon globe, such as used by truckers in their side clearance lamps. This needs to be shield by a coffee tin lid painted black and provided with a slit about an inch long and about 0.125 deep to reduce the
light to manageable proportions.
Australian facetor, Bob Davis, devised an even better stoneholder to avoid messing around with BluTak. Cut two strips of formica about three inches long by .75 of an inch wide and cement them together for about one third of their length. Drill the other
end with a three-sixteenth inch hole and your stones will fit neatly in the holder. If you cut large stones, then drill a larger hole to accommodate them.Facetors can inexpensively build up a reference set of faceted stones, labelling them with their
RI, degree of double refraction, dispersion factor and other information.
This system only works with faceted stones and variations will be discovered between the standard round brilliant and say, a long rectangular barion. The pattern of the spectra will vary according to the facets cut. The estimation of refractive index
improves with practice and experience with stones of varying colour density. Hodgkinson and Hanneman's publications will show how you can create a portable paper refractometer scale that easily shows the difference between quartz and topaz and of course,
can calibrate, CZ, YAG, strontium titanite, sphene and other materials above the range of the normal refractometer. The ability to arrive at a rough RI, the amount (if any) of double refraction (birefringence) and the degree of dispersion (fire) will
clearly identify most of the stones with which the average facetor comes into contact.
This is simply an inexpensive introduction to gemmology as it applies to faceting and very obviously has limitations. Very obviously the angles used in faceting play a part in the spectral display and the system works best with round brilliant cuts, but
with practice the average cutter will be able to accurately identify most of the faceted stones he or she comes into contact with and with varying gem shapes.
If nothing else, Visual Optics will give facetors an greater interest in the gemmological features of the materials they handle, and possibly, lead them on to greater studies.
BIBLIOGRAPHY:
"Visual Optics - Diamond and Gem Identification Without Instruments" published by Gemworld International Inc., of Northbrook, Illinois 60062, USA. ISBN 0-9641733-1-X.
Educating the Eyeball—The Hodgkinson Method. W. W. Hanneman PhD Lapidary Journal October 1980. (please note the colour plate is printed upside down and back to front. Peter Collins has made up a correction slip for this page).
The Art of Gemmology. Three part series explaining how Visual Optics works. W. W. Hanneman Ph.D. Lapidary Journal Sept, Oct and November 1991
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Copyright, 1998 by Peter Collins
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Peter Collins was co-founder of the Australian Facetors' Guild in 1981 with his friendly pharmacist, Ray Sainsbury. What started out as a quiet weekend for a few facetors has grown to the largest single organisation devoted to faceting in the
world.
Peter edited the AFG magazine, Facet Talk, from its inception in 1981 to 1993. He was a journalist by profession and was night editor of a daily provincial newspaper in Queensland where he retired. Peter served the AFG as president, secretary and State
delegate and currently runs the guild's video-tape library which carries more than 120 titles. Members of the AFG and lapidary clubs can borrow the tapes for a minimal charge to cover outward postage or they can buy copies of tapes.
In 1986, Peter was one of the architects of the International Faceting Challenge which is conducted every second year. The USA won the first round and Auastralia has won the rest with the 1998 round to be decided this Easter at Gawler, South Australia, as
part of the country's largest gem show, the Gemboree.
He played a major part in the establishment of the United Kingdom Facet Cutters' Guild. He and Heather, his wife, spent part of their honeymoon in 1994 convincing a handful of facetors that the UK must have more cutters. Today the UKFCG has over 100
members. Peter uses a 14- year-old Raytech-Shaw machine and tries to cut almost every day. He is Australia's leading archivist of faceting diagrams, having a collection of almost 6000. He, like many others, seeks someone to fill the shoes of the late
Norman Steele to continue the task of cataloguing new faceting diagrams as they are published.
With John Broadfoot, another former president of the AFG, Peter is almost finished writing a comphensive faceting manual. They hope to have it published in the first half of 1998.
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