Undoubtedly, the most famous detective in fiction is Sherlock Holmes. In the Doyle stories about Holmes, the key to his effectiveness was his surpassing keenness of observation and perception. Gem identification is detection in a sense. Similarly, the key to skillfulness is a combination of visual acuity and keenness of perception. Instruments are essential to accuracy, but more errors are avoided by a careful unaided-eye examination of a stone than by any other phase of the identification procedure.
The effectiveness of the initial examination is related directly to the experience and knowledge of the observer. When a sufficient number of stones have been studied thoroughly, characteristics such as luster, fractures and cleavages, inclusions, birefringence, color, weight by heft, and dichroism provide valuable information. Together, they often permit the expert to be sure of the identity of a stone without using instruments.
The student, of course, cannot hope to utilize fill of the valuable visual characteristics of a given stone until he has handled a sufficient number of each species and variety to become familiar with them. Even then, visual examinations should correctly serve only as a guide to select the most effective sequence of instrument tests, and any apparent property should be verified with instrumentation.
The question may arise "Why not start with instruments as the first step?" The answer is that each gem-testing instrument provides limited information. For the broad picture, an initial examination by unaided eye is essential. A number of instruments may be used without filling gaps in the cabin of evidence gathered. For example, blue stone may show a 1.76-1.77 R.I. and a 4.00 S.G, conforming that it is Sapphire. It may, however, be a Sapphire doubted and only visual examination will reveal its true nature. Similarly, a mounted stone may provide an R.I. that seems to suggest a given species, and yet, because the facet tested is slightly rounded due to poor cutting, the index and subsequent identification may be in error. Care full examination would have revealed the curvature of the surface and provided a warning that a quick routine test might not yield correct information. Again, observation and perception, coupled with careful instrument work, are the requirements for accurate identifications.
It must be realized, however, that visual examination involves only the stone itself and not the often misleading story provided by the appearance of the mounting, the apparent net worth of the owner, suspicious behavior, etc. For example, at one time a large loan organization posted an employee at the entrance to their building to inform the firm's appraisers of the kind of conveyance in which a customer arrived. A limousine suggested that the stone must be natural and the appraisal should take this into consideration. Unfortunately, jewelers are still influenced in their identification work by similar factors; e.g., assuming that a stone set in platinum with diamonds must be natural, or that one mounted in silver or even base metal must be synthetic or imitation.
For many years, experts recommended a procedure that required after initial examination, that all possibilities be listed-on the basis of color. After completion of each of the recommended tests, eliminations from this list were made, until only one stone remained. Identification was made merely by a process of elimination. Although this method was rather effective for teaching the novice, it had two major drawbacks: the preparation of the list proved too long and tedious to be practical, and major eliminations could not be made until late in the series of the tests.
To reduce the time required to make an identification, three steps should be followed. The information thus accumulated should clarify the extent or the direction of testing required beyond that point. These three steps are :
- initial examination by the unaided eye,
- a more thorough examination of the stone's interior under magnification, and
- an R.I. reading, if the stone surface is sufficiently well polished to permit this to be done by refractometer.
The major purpose of this assignment is to provide instruction in what to look for in the unaided-eye and low magnification examination.
What characteristics of gems that are visible to the unaided-eye give important clues to identify or suggest courses of action to follow in further testing of establish identity ?
- Strong doubling, resulting from strong birefringence, immediately proves double refraction and indicates the possibility of a stone with an index above the limits of the refractometer; for example, zircon or synthetic rutile. Such stones are frequently misread on the refractometer, since the student generally is looking for, and expecting, a reading and thus mistakes a faint shadow edge (which might be merely a boundary of the stone's surface on the hemisphere) or a complete spectral line, including red (produced by light from an overhead source being dispersed through the stone) for a reading.
- Strong dispersion. This usually indicates an R.I. above the limits of the ordinary refractometer.
- Characteristic fractures. A translucent or opaque stone that displays a conchoidal, vitreous fracture cannot be a crystalline aggregate, since the small grains of which it is composed prevent the occurrence of a smooth, glasslike surface on a break. Similarly, a dull or granular fracture cannot occur on a stone cut from a single crystal. These characteristics are important in the case of glass limitations of jade, turquoise, chalcedony and other crystalline aggregates, since the density and R.I. of the glass may approximate that of the natural counterpart.
- Concave facets or those with a surface resembling that of an orange cannot be produced by usual polishing methods, but they are typical of molded imitations. Refractive indices from such a surface may be indistinct and unreliable. In Figure 3, the oval-shaped shadows on some of the facets indicate their concave nature.
- Numerous inclusions that produce a translucent effect make a poled- scope analysis questionable.
- Abrasions on polished surfaces should always be noted and care used on testing such a surface on the refractometer. This is a particularly true of a cabochon with an abraded apex. The sharp edges of the abrasions can easily damage the soft glossy hemisphere. Usually, such stones will nave a satisfactory surface nearer to the girdler.
Although visual examination and handling of a stone are essential procedures prior to instrumentation, the commercial value of examinations without instruments should not be under estimated. It often provides experienced gemologists with the information necessary to make very advantageous purchases or to avoid costly and embarrassing mistakes. For example, a gemologist student was once offered a large carved blue stone as quartz by an antique dealer. He hefted the stone and also noted the luster, deduced immediately that it was not quartz, and made the purchase for a very nominal price. A subsequent instrument check proved it to be sapphire. Both the density and luster of the stone were much too great for quartz, even though it was offered at a price that would be expected for quartz.
On one occasion, a gemologist received from a well-known colored stone importer a group of fine cat's eyes, while examining the parcel he noticed that one stone seemed to be much too large for the weight shown on the paper. A quick check in heavy liquids indicated an S.G. similar to that of quartz, and a further test confirmed his suspicion. There was no obvious difference in appearance between this fine quartz cat's-eye and the chrysoberyls; however, to the keen eye of the gemologist, it was too large for the weight indicated to be a chrysoberyl. It is interesting that the stone had been offered as chrysoberyl by the importer for some time, and neither he nor any of the jewelers who saw it suspected its true identity.
Frequently, an experienced gemologist will encounter a stone that does not "look right". Although this suspicious appearance is often difficult to describe, he will notice slight deviations in luster, dispersion, texture or other characteristics that affect the appearance of the stone to the unaided but trained eye.
While talking with a colored stone importer, if gemology instructor questioned several beautiful stones that were labeled jade. After a quick check, they proved to be green grossularite, much to the surprise of the importer. Frequently, pawn shop operators and antique dealers purchase valuable stones for little more than the value of the gold in the mountings, never realizing their true worth. To an alert and experienced gemologist, an unaided-eye examination should provide at least an indication of a stone's identity.
The next test depends on the findings of the initial examination. If the characteristics observed limit the possibilities to a few stones, or perhaps just two, one test may be sufficient to make a positive identification; this is unusual, however. The usual second test (and the first instrument test) is examination under magnification. Although a loupe or monocular microscope is useful for this purpose, the most effective instrument is a binocular microscope equipped to provide dark field illumination in conjunction with immersion. It has the advantages of more efficient illumination, erect images, and stereoscopic vision.
The advantages of being able to study both the surface and interior of a stone under magnification, regardless of its nature, are many. It often discloses natural versus man made origin. The identity of a stone is sometimes suggested by characteristic inclusions, but they seldom provide conclusive evidence. Crystal habit may be indicated by the orientation of inclusions. If doubling of the back facets is detected, it proves the existence of double refraction and makes it possible to estimate the strength of birefringence.
The discovery of the new gemstone, taaffeite, bears testimony to the remarkable acuity of a gemologist with very limited equipment, except for a binocular microscope. Count Taaffe is an Irish gemologist who, while engaged in selecting a number of stones from the "junk box" of a Dubling jeweler, had separated the stones in which he was interested into various colors. Those he had sorted on the basis of luster, dispersion, polish and other properties visible to the unaided eye were first cleaned thoroughly and then examined, using the following equipment:
- methylene iodide, a hand scale for S.G. determinations.
- Polaroid plates
- a 21x binocular microscope.
One light violet stone weighing between one and two carats sank and in the methylene iodide rapidly, and because it nearly disappeared in the liquid, Taaffe assumed it to be spinel, which is singly refractive.
Examination under magnification, however, disproved this assumption, since a slight doubling of the back facets was observed. He verified this apparent double refraction by checking the stone between polaroid plates and found that it became alternately light and dark, the reaction one would expect from a doubly refractive material. Since this did not seem to correspond to any stone he knew, he checked the specific gravity, using the hand balance; the average of ten determinations thus made gave him a figure of 3.62. Since each test had indicated spinel, except for the presence of birefringence, Taaffe sent it to B.W. Anderson at the gem testing laboratories of the London Chamber of Commerce. There, Taaffe's determinations proved to be correct : accurate refractive indices were 1.718 and 1.723, and a very accurate S.G. determination resulted in a figure of 3.613.
Perhaps the most amazing aspect of this story is the fact that the S.G. was determined with a small hand held balance, and that the results were remarkably close to those obtained with fine equipment. It is obvious from this account that the extent to which a gemologist uses the equipment at his disposal determines his effectiveness in gem testing.
Characteristics of simple lenses
The magnifying power of a simple lens can he determined by dividing ten by the focal distance in inches. For example, if the sunrays were brought to a focus by a lens at two inches, the lens would have a magnification of five times. A one inch focal distance is characteristic of a 10x magnifier. The working distance is approximately the same as the focal distance. Depth of field (i.e., that portion of an object in focus at a given time) is inversely proportional to the number of magnifications; in other worlds, the depth of field of a ten-power lens is only a fraction of that of a three-power lens. Therefore, the factors to consider in a magnifier include working distance, depth of field of and, of course, strength of magnification.
One important consideration when using a loupe to examine gemstones is the practical limitation in magnification. For example, to go from 10x to 20x in a loupe means a reduction in working distance of 50% (from one inch to one-half inch), and also a similar reduction in field size. This close position of the lens to the object increases materially the lighting problem. A 20x loupe is difficult for anyone but a skilled person to use. Although 30x loupes are made, they are rarely used. It is advisable to examine a large stone or jewelry piece first with a low-power loupe with a wide field and to use one of higher power only to locate and study characteristics that require closer inspection.
A simple lens is subject to image distortion and to both chromatic and spherical aberration (Figure 6). In order for a lens to be of maximum value, these problems must be overcome, which may be done in one or more of several ways. Distortion is caused by the failure of a lens to bring into focus all points on the object at the same point. It is corrected by using multiple-lens magnifiers or by sandwiching lenses of different kinds of glass into single elements. If a lens is composed of two parts it is called a Doublet; if three, a Triplet. Chromatic aberration causes the various wavelengths of light to be brought to focus at different distances from the lens, so that objects have color fringes when viewed through the lens. This condition is corrected by using additional portions of different glass, which has the effect of reversing the dispersion and bringing the wavelengths of the various colors into focus at the same distance from the lens. If a lens is corrected for spherical aberration and a hazy border (coma), it is called Aplanatic (ap-lah-NAT-ik); if it is corrected for chromatic aberration, the name Achromatic (ak-roe-MATik) is applied. Different types of lenses arranged in series may largely correct for spherical and chromatic aberration and distortion.
Preparation for Loupe Examination.
Surface dirt or scratches can be confused easily with inclusions, especially when using a single magnifier, since it fails to give depth perception; therefore, a stone should always be cleaned carefully before examination. A loose stone can be cleaned adequately by rubbing it vigorously between the thumb and forefinger with a paper facial tissue or a silk cloth. If it is mounted and/or fails to respond to this simple treatment, an alcohol cleaning solution may be required. To remove dirt and grease from a mounted stone, usually it is necessary to use a detergent in lukewarm water and a small stiff-bristled brush, after which the piece should be rinsed with lukewarm water.
Steam cleaning may be used to advantage in many eases, keeping in mind that some stones are very susceptible to temperature change. When used properly, this method is one of the better and more thorough, however, extreme care should be exercised. Although this popular boiling out and steam method is widely used, several cases of severe damage have occurred. It has been reported to that an exceptional peridot was cleaned by immersion in vigorously boiling water; the result of course, was a badly fractured stone.
Many stones such as emerald and sapphire are oiled to mask fractures or improve color. When boiled and steamed, the oil is driven out, resulting in the original state of poor color or severe fracture.
In short, it is apparent that a detailed knowledge of optical and physical properties of each gemstone is required before undertaking any cleaning method. Cautionary steps to be taken follow:
- Consult property tables tor the stones reaction to temperature change.
- Examine the stone thoroughly, under magnification to disclose any fractures or cleavages, or evidence of surface coloration or oiling.
- Start the stone in lukewarm soapy water and bring the solution to a boil; this allows the stone to adjust to the temperature change on a gradual incline.
- Submit the stone to a rapid blast with steam to drive out water and residue; allow the stone to cool normally at room temperature.
After the stone has been cleaned, it is grasped at the girdle with a pair of tweezers and the breath is blown sharply at it to remove any lint that remains. Crushing lightly with a clean camel's hair brush may be necessary to remove any remaining lint or other foreign particles; this may be facilitated by the use of a sharp-pointed object such as a needle.
Because inclusions must be examined minutely, and because it is sometimes necessary to diagram them on paper, it is advisable to hold the stone steadily and securely. If locking or tension-type tweezers are not available, a rubber band wound around a conventional pair will serve this purpose adequately.
Now that the stone is ready to be examined, lighting is the next consideration; it is nearly as important for the loupe as for the microscope. If the light is placed behind the stone, the facets reflect it away, so the center may be dark and the eye blinded by the glare coming around the stone (Figure 7). The stone usually appears nearly opaque, yet this ineffective method is the one commonly used for lighting an object to be examined by a loupe. In order for inclusions to be seen most readily, they should be illuminated in a manner that makes them stand out as bright objects against a dark field. Thus, the stone needs to be lighted from the side and examined over a dull-black or other dark background (Figure 8). It is likewise important to shield the light from the eyes of the observer.
One effective illuminator for loupe examination is the ordinary case neck lamp with a metal reflector or a fluorescent type desk lamp. The lamp is directed downward, so the stone can be held at the edge of the reflector (Figure 9).
This illuminates the stone but keeps the direct rays from the eyes and the facets facing the observer. Still More efficiency wilt be gained if the area below the store is a dull black and normal overhead is reduced as much as possible.
Examining the Stone
A faceted stone usually is examined first through the table, since this large facet affords the clearest view of the interior. However, it should be examined from all directions, unless the first view provides all of the answers being sought. If the purpose is to plot all visible characteristics, a thorough examination is essential; for example, the stone should be held in the tweezers or the stone holder between the table and the culet, to permit an unobstructed view of the girdle. An all-direction examination often reveals color banding, pleochroism, faint, separations and other features that may be invisible through the table. If the stone is turned slowly as it is viewed, characteristics may be revealed that are not otherwise visible; this also helps the viewer to distinguish between surface and internal objects.
There are several means involving the use of the magnifier to determine positively whether an object is within a stone or on its surface.
- If a stone is turned about its center, an object below the surface will turn with a different are than one on the surface. Since an inclusion is likely to be closer to the hub of the turn, it turns in a tighter circle than one on the surface (Figure 10).
- The plane of focus is such that a comparison between the points around the crown or pavilion that are in sharp focus will show whether the object itself (which also is in focus) is between the two in-focus points at opposite sides of the stone or between two in-focus points on adjoining facets (Figure 11). If the object in focus is between opposite points in focus entirely across the stone, it follows that the object must be within the stone. If it is seen between adjoining facets, in all probability it icon the surface.
- This may be confirmed by turning the stone so that light is reflected directly from the facet on which the object may be resting.
If the object is on the surface, it should stand out in the direct illumination. One of the greatest advantages of a binocular microscope is the ease with which an object may be located precisely on the basis of the stereoscopic effect and the depth perception.
The novice gem tester tends to expect any internal characteristic of a gemstone to be disclosed readily by magnification. Often this not true, for the stone must he lighted in one manner to view inclusions and in another to disclose the nature of its coloring. For example, color bands do not become evident unless observation is made parallel to them (i.e. like looking at the edge of a deck of cards) with the light coming; not from the side, as recommended for inclusions, but from beneath the stone. For this reason, when a resolution of color banding is sought, examination with a very diffused light source placed behind the stone is recommended. Merely placing the stone over a lamp often throws so much light around the edges that the true nature of the stone is obscured; therefore, the intensity of the light must be reduced. If the gemolite or diamondscope is used, a paper facial tissue placed over the light opening will produce the necessary diffused background. Sometimes, particularly when working with a tiny stone, immersion in a liquid will reveal its characteristics more readily. This method is particularly advantageous when the tester is attempting to distinguish between curved striae and straight color banding. Occasionally, it is necessary to increase the contrast between light-and dark-colored zones by diffusing the light as indicated above. This may be accomplished by placing the tissue under the liquid container or using either a frosted glass container or the type of cold-cream jar made of opal glass. An immersion cup to be used on a gemolite or diamondscope or with another magnifier should have very low sides; this facilitates holding the stone in the tweezers or moving it about. (Note : The spring loaded stone holder used on the gemolite and diamondscope should not be immersed in liquids). The subject of immersion is discussed at length elsewhere in the course.
One of the most-valuable aids provided by effective magnification is the detection and estimation of the strength of birefringence. Detecting double refraction in the form of doubling, either of back facet junctions or of dust and scratches on the opposite side of a stone, avoids the necessity of using the polariscope for this purpose. Since the polariscope determination is more subject to error and misinterpretation than most other tests, this is a very worthwhile determination. Moreover, the polariscope, when used in the ordinary manner, merely determines the presence of single or double refraction, whereas the detection of doubling and the determination of the width of separation between the two images provides not only proof of double refraction but a measurement of its strength. In addition, the time required to use the polariscope is saved.
Initial recognition of doubling to be one of the most difficult subjects to convey adequately to beginning students. Once detected, however, this property is recognized easily in the future.
Calcite, a small specimen of which was furnished with the Colored gemstone course, is an ideal material with which to become familiar with the nature of doubling. Looking through the calcite to its opposite side with the loupe shows clearly that each of its edges appears twice; in other words, a double image of any feature appears when seen through the stone.
The next step is to use the loupe to locate and become familiar with the appearance of doubling in a zircon. Since zircon may be cut so that the optic axis is perpendicular to the table, little or no doubling may be visible near the culet when viewed through the table; therefore, if it is not noted immediately, examination should be made through the bezel facets. It is important to look through a single facet, rather than through two different facets.
Doubling in a stone that has a birefringence of lower magnitude than the .172 of calcite or the .059 of zircon (e.g., the .009 of quartz or the .008 of corundum) is more difficult to see initially with a loupe, unless a very large stone is being examined. After its appearance has become familiar, however, and when every dust particle and minute scratch on the opposite side of a stone can be seen as a doubled image, it saves the gem-tester's time, ensures a higher degree of accuracy, and improves his all-around testing ability. The amount of doubling is directly proportional to a stone's strength of birefringence and to its size. Figure 12 & 13 shows the relative doubling observed in corundum 12, tourmaline 13, blue zircon (C), and synthetic rutile (D). All were photographed under 30x. The lower the birefringence and the smaller the stone's size, the greater is the difficulty in detecting the phenomenon, for the width of separation is small.