Gemmology On A Shoestring

(via The Journal of Gemmology, Vol.10, No.3, July 1966) B W Anderson writes:

Further simple tests
The tests so far suggested have involved only the use of lens and tongs. I am now going to suggest the use of a few very simple ‘extras’ which I feel can be included under our ‘shoestring’ limit, and which will undoubtedly extend quite considerably the scope and certainty of our gem identification. The first extra is the well-known Chelsea filter, which was developed by A Ross Popley from a formula worked out by C J Payne and myself in the early thirties. In knowledgeable hands, and properly used, the filter can be extremely useful. If used without any knowledge of how it functions, it can be quite misleading. Basically, it is a very efficient ‘dichromatic’ filter, transmitting only a narrow band of deep red light and a narrow band of yellow green. Thus, through the filter, a stone can only appear red, green, or a mixture of the two. Filters of this type were originally designed to distinguish between emeralds and pastes or doublets. Emerald, unlike most green stones, both transmits deep red light and emits fluorescent red light when it is brightly illuminated. To see the effect at its best, the stone should be held immediately under a good tungsten light and viewed through the filter held close to the eye. The snags in this simple use of the filter for emerald are that two other green stones, demantoid garnet and fluorspar, may show a reddish residual color when viewed through the filter, that synthetic emeralds appear an even more decisive red than natural emeralds, and that natural emeralds containing enough iron to damp its fluorescence and cause absorption in the deep red do not appear red through the filter. But in my opinion the warning given by the very intense red shown by Chatham synthetic emerald when viewed through the filter is a most useful indication, white its other uses in clearly distinguishing between aquamarine (green appearance) and synthetic blue spinel (orange red) between stained green chalcedony and chrysoprase, etc. serve to add to its value.

Next, quite another kind of filter: Polaroid. This astonishing invention of perhaps the most inventive of modern men, E H Land has placed polarized light, with all its peculiar and revealing properties, within the reach of everyone, and in a form far more compact and convenient than the old Nicol prism. A number of different formulae have been employed in producing the highly dichroic substances in plastic sheets which constitute Polaroid, but the effect in each case is essentially the same—that a ray of light which has passed through such a sheet is vibrating parallel to one direction only—that is, it is completely polarized. Such pieces of polarizing film are capable of far more valuable and fundamental used in the study of gemstones than any color filter can be, and fortunately the material is quite inexpensive; about four shillings per square inch, up to virtually any size required. The most favored type transmits 34% of incident white light, and two pieces of this in the ‘crossed’ position give virtually complete extinction.

One of the most obvious ways of using this material in gem testing is to mount two discs of polaroid in the ‘crossed’ position with a space between them enough to accommodate any gemstone, thereby forming that very sensitive instrument for the detection of double refraction known as the ‘polariscope’. A gadget of this kind can be easily home made, but there are some inexpensive types, carefully designed for convenience in use, which are commercially available. A useful pocket polariscope is one made by Rayner from a design of Dr E Rutland’s, while the same firm make an extremely convenient table model with a built-in light, which leaves ample room for large specimens, either dry or in a cell of suitable liquid; it can also easily accommodate massive pieces of jewelry, stones in necklaces, etc. can be examined, and both hands are free for manipulation.

A gemologist worth his salt will get much more information from such a polariscope than ‘four times light, four times dark—there a doubly refracting crystal’. He will learn to recognize the characteristic types of ‘strain birefringence’ shown by paste imitations (which often show a crude interference cross) by synthetic spinels, with their ‘tabby extinction’ patterns, by diamond, which is never truly isotropic in gem sizes, but shows brilliant interference colors, usually centered on the various points where tiny inclusions can be located, and so on. He will also learn to use a pocket lens to reveal interference figures, at least in the simpler cases, and the sight of a uniaxial figure through the table facet of a ruby or sapphire will give him fairly strong assurance that the stone is a natural one. The unique interference figure of quartz, with its hollow colored center can often provide a quick proof for this mineral. This is beautifully and easily seen in beads of rock crystal or crystal balls, in which the spherical shape makes the figure visible without the use of a lens to provide the ‘conoscope’ effect.

Polaroid can also be used to detect dichroism in gemstones. Two pieces can be set with vibrations at right angles to each other in a pair of old spectacle frames, and a specimen viewed in rapid alternation first with one eye and then the other, often showing a marked change in color, or narrow strips of Polaroid in alternating vibration directions can be counted on a disc of plain glass which can be mounted at one end of a short metal tube with a low power lens at the other. This forms an effective dichroscope for stones that are not too tiny: a dichroic stone viewed through the tube showing alternating stripes of different color or depth of color. Alternatively the Polaroid disc at the end of the tube can be cut into four sectors, the top and bottom sectors transmitting light vibrating, say, north and south, while the left and right hand sectors transmit only light which is vibrating east and west. In testing for dichroism it is best to use daylight reflected from a white surface if possible, and always one should turn both the specimen and the dichroscope tube before deciding on the strength of the dichroism—if there is any.

The next simple and inexpensive aids to gem testing that I want to recommend comprise liquids of various kinds, glass cells of suitable depth and diameter, a glass funnel and some filter papers. These can be used in a number of ways: to enable the color distribution and other internal features of rough or cut stones to be studied with ease: as a rapid test for the density of unmounted stones: and as a means of assessing the refractive index of unknown specimens.

A useful stock to begin with would be two ounces each of methylene iodide, bromoform and bromobenzene, and four ounces of monobromonapthalene, each in screw-top bottles of brown glass.

When stones are immersed in a liquid of similar refractive index the surface reflections and the effects of refraction are largely eliminated and one is able to ‘see into’ the specimens as easily as though they were parallel-sided plates. The ‘frosted’ effect of the surface of rough gem pebbles is also eliminated when they are immersed in a suitable liquid, and lapidaries are well advised to use this means of seeing the color distribution, flaws, etc. of rough gems to enable them to be cut to the best advantage.

Gemologists are familiar with the technique of placing a suspected ruby or sapphire in an immersion cell of liquid before examining it under the microscope, but they may not realize how helpful immersion may be for observations with the naked eye or with a lens. Sapphires in particular can usually be recognized as natural or as synthetic when immersed in bromonapthalene and viewed against a white background. Natural sapphires almost invariably show zones of color with rigidly straight edges, while synthetics slow the well-known curved swathes of color when observed in the correct direction.
A simple and effective means of illumination is to place the stone in its immersion cell on the base of another, inverted glass cell of rather larger size, and direct the light from a bench lamp on to the white blotting paper on which this stands.

For density tests a stone must of course be free from its setting. Granted this, there is no simpler or more decisive way of distinguishing, for instance, between chrysoberyl and quartz cat’s eye, between aquamarine and synthetic spinel, or topaz and yellow quartz, than to put the stone in question into a tube of methylene iodide and seeing whether it floats or sinks. On the whole it is advisable to keep your liquids as pure compounds rather than as mixtures, as in that way their densities and refractive indices remain constant, except for slight variations with temperature. A very useful mixture, however, is one of bromoform diluted with monobromonapthalene until a small clear quartz crystal remains suspended in it. So constant is quartz in its density that this will serve as a virtual identification liquid for any of the transparent quartz gems such as amethyst and citrine. But also it will serve to identify Chatham, Gilson, or Zerfass synthetic emeralds, since the density of these is almost identical to the 2.651 of quartz.

Using liquids as a guide to the refractive index of gemstones has much in common with their used in checking density. In both cases a quite crude test may be all that is required to resolve a doubt, and in both cases quite accurate results can be achieved if this is necessary by taking more time and refining one’s techniques. Even with mounted stones liquids can quickly give useful clues to refractive index. If the small diamonds in a cluster ring, for instance, are suspect, a clear decision can be given if the entire ring be immersed in methylene iodide and the stone viewed with a lens. If the stones are diamond the refractive index is so much higher than that of 1.74 of the liquid that the facets and edges will still appear sharp and clear, while synthetic white spinel and synthetic white sapphire will virtually disappear in this fluid. With loose stones, a very fair idea of their refractivity can be quite quickly and easily obtained by placing the stones table facet down in fair-sized glass cell and pouring in enough monobromonapthalene (R.I = 1.66) to just cove them completely. The cell should be placed on a sheet of white blotting paper and the stones viewed by the light of a single bulb some distance overhead. Those stones with an index higher than that of bromonapthalene will show a distinct dark rim round their periphery, as seen projected on the paper below, and the projection of the facet edges will appear white whereas with stones of lower refractive than the liquid a pale surrounding rim can be seen with the facet edges as dark lines. The degree to which the index of the stone is lower or higher than that of the liquid can be assessed with fair accuracy by noting the width of the dark or pale rim. Unlike the well-known “Becke’ methods for gauging whether an immersed grain is more or less refractive than the liquid, the procedure leaves no doubt at all in the observer’s mind about which has the higher index. A refinement of this simple test is to place the cell containing the immersed stones on a finely-ground glass sheet which is made to act as a bridge between two four-inch blocks of wood or still cardboard. A ‘handbag’ mirror of suitable size is placed at 45º under the cell, enabling the projection onto the glass sheet of the stones, to be seen in detail and in comfort. The effects seen are really very beautiful as well as revealing. If a permanent record is required, a contact photograph can be easily obtained in a dark room by placing the cell containing the stones over a piece of slow film and exposing for a few seconds to an overhead light. Those who are photographers can make use of the narrow beam of light from an enlarger stopped down to f22, and thus obtain very sharp and detailed photographs. Internal features of the stones, in particular color zoning, show up very clearly on such photographs, particularly where the liquid and stones of nearly the same index. I have found that the curved striae in synthetic corundums may be discerned in such photographs even if invisible under the microscope. For this, however, carefully filtered methylene iodide is necessary, and one must be lucky in hitting the right direction for showing the lines. I have only had time to give bare outline of these methods: but anyone seriously interested will find details, with diagrams and photographs, in my book ‘Gem Testing’.

I have included bromobenzene in my short list of useful liquids because this fairly pleasant and inexpensive liquid as a refractive index of 1.56. This makes it an ideal immersion liquid for the critical examination of emerald: it enables the thin rim of dark color to be seen at the edges and corners of the ‘Lechleitner’ type of synthetic emerald, in which a pale, faceted beryl is used as the ‘seed’ on which a thin crystalline layer of synthetic emerald is grown hydrothermally. Quite a few of these stones are now being used in mounted jewelry. The index of bromobenzene is also between that for synthetic emeralds of the Chatham, Gilson and Zerfass types and the indices found in natural emeralds. A good immersion contact photograph of synthetic and natural emeralds immersed in bromobenzene will reveal this.

Two pieces of advice I should like to add because I am so convinced of their importance for any budding gemologist. The first is to start a collection of gemstones, because the ability to compare the appearance and properties of an unknown stone with known samples is of inestimable value in testing. Even a collection of all available synthetics, doublets, pastes, etc. makes a most useful and interesting beginning. For genuine stones the ‘shoestring’ may only permit one to acquire only quite small specimens, or chipped or broken pieces, but one must realize that money spent on any fine specimen is not lost but invested, since the value of these is continuously increasing. The second piece of advice is to have at hand one or two reliable reference books. Two by Robert Webster stand out by reason of their accuracy and comprehensiveness. His ‘Gemologist’s Compendium’ contains all the necessary data gemstones in condensed form, and is very reasonably priced, while his two-volume work ‘Gems’, though expensive, contains such a wealth of information as to be well worth the money for any sizeable firm or jewelers or any serious student of the subject.

A trained gemologist can go a long way in gem identification by intelligent use of a good lens and few simple bits of apparatus, but I must repeat the warning that some of the problems confronting the trade today need more than this for their correct solution: they need all the facilities and skills of a specialized laboratory.