A SIMPLE PROCEDURE TO SEPARATE NATURAL FROM SYNTHETIC AMETHYST ON THE BASIS OF TWINNING
By Robert Crowningshield, Cornelius Hurlbzzt, and C. W. Fryer
Dr. Karl Schmetzer recently showed how ince 1970, when synthetic amethyst first became near-flawless to flawless natural amethyst savailable commercially, it has created problems in could be separated from synthetic ame- identification for gemologists. Although inclusions have thyst on the basis of the presence of Brazil proved reliable in distinguishing synthetic from natural in twinning in the natural stones. Whereas most "flawed" amethysts, until recently no test was avail- Dr. Schmetzer's procedure required a spe- able to separate the flawless or near-flawless stones that cial apparatus, the authors have deter- mined that a standard gemological po- represent the bulk of the fine faceted stones on the market lariscopeis more than adequate to make (figure 1).As a result, otherwise ethical jewelers and sup- the separation in most cases. Although pliers everywhere may have unwittingly sold thousands of some synthetic amethyst does show evi- synthetic amethysts that were represented to them as nat- dence of twinning, in the synthetic stones ural. In 1985, however, Dr. Karl Schmetzer described a examined thus far it has taken a form that procedure by which a distinction can be made (see also is distinctly different from the Brazil Schmetzer, 1986).Subsequently, the authors adopted Dr. twinning seen in most natural amethysts. Schmetzer's procedure for use with standard gemological The presence of certain inclusions as well equipment. This test alone is not always unequivocal but, as the nature of the color zoning seen in when used in conjunction with other observations, such as natural versus synthetic amethysts is of color zoning, it presents an excellent method for the sepa- primary use in making a separation. How- ever, where there are no inclusions or color ration of most near-flawless synthetic and natural zoning, the presence of Brazil twinning in amethyst. the natural amethyst will usually make The procedure described by Dr. Schmetzer is based on the distinction. the fact that most natural amethysts are repeatedly twinned on the Brazil law, while synthetic amethysts are usually grown as single crystals (Schneider and Droschel, 1983; Lind et al., 1983).In polarized light, a twinned stone will exhibit varying degrees of interruption in the spectral rings, while an untwinned stone will show undisturbed ABOUT THE AUTHORS rings of spectral colors. While Dr. Schmetzer's description Mr. Crowningshield is a director of the GIA Gem suggested that a special apparatus was required, we have Trade Laboratory in New York City; Dr. Hurlbut is found that a polariscope or standard polarizing microscope professor emeritus 01 mineralogy at Harvard Uni- versity, Cambridge, Massachusetts; and Mr. Fryer is adequate to observe the diagnostic twinning. To confirm is chief gemologist in the Research Department 01 the accuracy of our method, we examined more than 1100 the Gemological Institute of America, Santa natural and 200 synthetic amethysts. In this article, we Monica, California. describe how to apply these simpler procedures. Acknowledgments: The authors would like to thank Mr. Ross Altman, Ross Altman, Inc., and Mr. Paul First, however, it is well to review twinning in ame- Heubert, Inter-Ocean Trade Company, both of thyst as well as why synthetic quartz was made in the first New York, lor the gracious loan of stones lor study. place. Other tests useful in distinguishing natural from 0 1986 Gemological Institute of America synthetic amethyst are also discussed briefly.
130 Identification of Amethyst GEMS & GEMOLOGY Fall 1986 Figure 1. One of the major problems facing gemologists today is the separation of naturalfroni synthetic ame- thyst, as illustrated by the 9.40-ct natural oval cut and the 5.48-ct synthetic emer- ald cut shown here. Photo 0Tino Hanimid.
TWINNING IN AMETHYST terminalr or rand 2 faces. The lamellae are alter- The fact that almost all natural amethyst is nately right- and left-handed. They may give rise to twinned according to the Brazil law has been sets of delicate striations, or to open polygonal known for at least 150 years. The optical structure markings on the rhombohedra1 faces, and cause a of amethyst was first described by Sir David rippled, or fingerprint appearance on the fracture Brewsterin 1821 (Frondel, 1962),and was correctly surfaces. The twinning can be studied in etched interpreted as due to the polysynthetic twinning of sections or, more conveniently, by optical means." right- and left-hand quartz. In the course of the The last sentence of the above quotation, if 19th century, further descriptions were given by applied to the study of gemstones, might better many other workers. A brief summary of these and have been written: "The twinning can be studied later works is found in Dana's System of Mineral- in etched sections or more conveniently and less ogy (Frondel, 1962): "Amethyst virtually always destructively by optical means." For etching is shows polysynthetic twinning on the Brazil law. done with hydrofluoric acid, a harsh treatment of a Untwinned crystals have been noted. The twin gem. The twin lamellae are, however, made read- lamellae, a fraction of a millimeter thick, are re- ily apparent by the process (figure 2). markably uniform and are arranged parallel to the Gemologists are familiar with the term poly-
Identification of Amethyst GEMS & GEMOLOGY Fall 1986 131 synthetic twinning in connection with the re- amethysts have been observed, they were probably peated twinning of corundum and some feldspars. cut from the area completely under the minor In the termpolysynthetic, "synthetic" is not to be rhombohedron. confused with its use meaning "man-made," but rather carries the classical meaning of "put to- THE HISTORICAL DEVELOPMENT OF gether;" while "poly-" is a combining form mean- SYNTHETIC AMETHYST ing "many." Thus, the term polysynthetic con- Before World War 11, Brazil was able to supply the veys the meaning of "many thin crystals (lamellae) world with the rock crystal necessary to make the put together." oscillators used to control radio frequency. Since Quartz is optically active; that is, if polarized the oscillator must be free of twinning, the first light moves parallel to the optic axis, the plane of step in manufacture was to inspect the raw crys- polarization is rotated. The rotation is to the right, tals for Brazil twinning. This was done in an im- conventionally clockwise, in right-hand crystals mersion tank, using a giant polariscope with polar- and to the left in left-hand crystals. The shorter the izers one foot square. The hand-held quartz crystal wavelength, the greater is the rotation; violet light was turned until the optic axis was parallel to the is thus rotated more than red light. As stated in the direction of light through the polariscope and the above quotation from Dana's System of Mineral- Brazil twinning, if present, was observed. Many ogy, Brazil twinning results in alternating lamellae crystals with twinning throughout were discarded, most commonly under faces of the major rhombo- others with little or no twinning were passed on to hedron, r, the large triangular faces that terminate the next operation, cutting. the quartz crystal. Figure 3 is a drawing of a section During the war, however the demand for Bra- perpendicular to the optic axis that shows twin- zilian quartz skyrocketed. In the United States ning under the major rhombohedron with the alone, thousands of tons of quartz crystals were right-hand portions of the quartz shown in white used in the manufacture of over 50 million small and the left-hand portions of the quartz shown in oscillator plates cut at precise crystallographic black. If slices of quartz represented by this draw- angles. In 1944, because quartz was high on the ing were observed between crossed polarizers, they critical list of minerals, the U.S. Signal Corps initi- would take on the appearance seen in figure 4. One ated a quartz synthesis program. Private industry would see narrow parallel bands of alternating also became involved. Although success was not colors and shadow under the major rhombo- achieved until after the war, by 1950 hydrother- hedron~and broad swaths of color under the minor mally grown synthetic quartz was in mass pro- rhombohedrons. duction. Now untwinned quartz crystals grown on With light moving parallel to the optic axis untwinned seed plates supply the material for os- through the twinned areas, the rotation of polari- cillators in radios, watches, cloclzs, radio- zation is alternately right and left. Because the frequency filters, and other apparatus where fre- lamellae are not of exactly the same thickness, the quency control of electrical circuits must be net rotation is always greater in one sense than precise. the other, resulting in different colors for the The successful synthesis of colorless quartz right-hand and left-hand regions. The lamellae are pointed the way to the manufacture (by the irradi- thus striated and sharply defined, whereas the ation of iron-bearing synthetic quartz) of colored untwinned areas are of a single color. Twinning quartz (Balitsky, 1980).But with citrine and ame- may be confined to isolated triangular areas, but thyst so abundant and relatively inexpensive, frequently in amethyst it will be evidenced by there would seem to be no incentive to synthesize straight lines crossing the stone. There may be these materials. Since about 1970, however, both only one set of parallel lines, or there may be two or have been made commercially in the Soviet Union three sets making angles of 60' or 120' with one and, later, in Japan. To date, most synthetic pro- another (again, see figure 4). These latter result duction is untwinned, single-crystal material. from the meeting of the twinned areas under two or three of the rhombohedron faces. DETECTION OF BRAZIL TWINNING In the authors' experience, untwinned crystals WITH THE POLARISCOPE (as noted in the Dana's quotation above) are very It is to Dr. Karl Schmetzer, of the University of rare. Although some natural untwinned faceted Heidelberg, West Germany, that credit must be
132 Identification of Amethyst GEMS &. GEMOLOGY Fall 1986 Figure 2. Etching an ame- thyst with hydrofluoric acid clearly reveals [he presence of Brazil twinning in these two 18-ct stones. The test is, however, highly destructive and therefore not practical for gemologists. Photo by PShane McClure. given for first promoting (1985)the optical test to useful. Since amethysts tend to be relatively larger differentiate flawless or near-flawless synthetic than more costly stones, we have found small from natural amethyst. However, there is the im- flared colorless glass votive candle holders, about plication by Schmetzer that to observe the twin- two inches (5cm) high and two inches in diameter, ning, one must use an "improved sample holder" to be good immersion cells. They are also sold as with a horizontal immersion microscope. We have oyster cocktail condiment holders for restaurant found that twinning can be equally well observed use. One is shown with a polariscope in figure 5. using a vertical polarizing microscope, but even Various immersion liquids may be used, but the more easily by simply using a standard gemologi- closer the refractive index of the liquid is to that of cal polari$cope with the stone held in ordinary quartz, the better. However, water usually pro- stone tweezers or even in one's fingers. vides relatively good results. In adapting Schmetzer's procedure to the open To observe twinning in amethyst, the stone polariscope, we have used several items that are must be turned so that its optic axis is parallel to readily available. Many times, the twinning effect can be seen without immersion. However, when immersion is necessary, a number of items are Figure 4. The twinning evident in this natural amethyst closely resembles the idealized draw- ing shown infigure 3. Note the narrow bands of Figure 3. This idealized illustration shows left- color under the major rhombohedrons and the hand (black) and right-hand (white)portions of a broad swaths of color under the minor twinned amethyst. Adaptedfrom Schlossin and rhombohedrons. Magnified 15 X; Lang, 1965. photomicrograph hy Robert Kane.
Identification of Amethyst GEMS & GEMOLOGY Fall 1986 133 Figure 6. The differencebetween the twinned natural (oval) and the untwinned synthetic (em- erald cut) amethyst is clearly seen lookingin the optic axis direction of each under crossed polar- izers. Photo by David Hargett.
right direction. Another problem, more serious, is that if a natural stone has been cut so that it lies entirely under the minor rhombohedron, it will not exhibit twinning. Sometimes only a small area of the stone encompasses an area under the major Figure 5. A simple polariscope and immersion cell are sufficient to identify Brazil twinning in rhombohedron, but the diagnostic twinning effect, near-flawless to flawless natural amethyst. although minimal, is still conclusive. As quoted Photo by David Hargett. from Dana above, there have also been reported rare untwinned natural amethyst crystals. These could not be detected by the Schmetzer test alone. the line of sight through the polariscope or micro- Of course, mounted stones offer another problem. scope, which must have crossed Polaroids, that is, If the optic axis direction is obscured by the set- the dark position. The basic problem is thus one of ting, they cannot be tested without unmounting. crystallographic orientation. Cut amethysts usu- Herein lie the shortcomings of the test. ally are not oriented except for maximum weight Figure 7 shows the twinning seen with the retention, or best color, so that finding the optic polariscope in a variety of natural stones. Equally axis direction is a challenge. This is the reason Dr. good results can be obtained with a polarizing Schmetzer felt a special holder was necessary. microscope (figure 8). In contrast, figure 9 illus- Only rarely is a stone cut with the table perpen- trates three untwinned synthetic stones. The nat- dicular to the optic axis. Nevertheless, such is the ural stone shown in figure 10 had to be maneu- case with the two stones in figure 6. The oval is a vered quite a bit, as it apparently was cut from natural amethyst showing twinning, whereas the material predominantly under the minor rhombo- emerald cut shows the untwinned effect of a syn- hedron. However, even a tiny area of fine parallel thetic stone. The photograph was taken "dry," colors shows the presence of twinning, and thus that is, the stones were not immersed. given our present knowledge proves natural origin. Rarely in practice will the test yield such con- Some synthetic amethysts, notably from clusive and dramatic results as easily-and with- Japan, do exhibit twinning (Balalzirev et al., 1975). out immersion. For one thing, the optic axis of the However, our observations thus far indicate that stone being tested may coincide with the point of a the twinning in these synthetics has a distinctly fancy shape or the girdle of any stone, making it different appearance from that of natural ame- difficult to maneuver with the tweezers to find the thyst. The twinning in the synthetics appears as
Identification of Amethyst GEMS & GEMOLOGY Fall 1986 figure 7. As these three examples of natural ame- :hyst viewed through a polariscope indicate, the brazil twinning may vary somewhat from one ;tone to the next. Often the stone will have to be maneuvered with the tweezers or in one's fingers 'o see the twinning clearly. While immersion nay be helpful, these photos were all taken with :he stones dry. Photos by Shane McClure.
irregular-shaped, small arrowhead-shaped, or Figure 8. Twinningin natural amethyst can also flame-like areas (figure 11). Such stones will have be identified using a polarizing microscope with definite zones of yellow or dark purple that coin- crossedpolflrizers. Magnified 6x; photo- cide with these flame-like or irregular-shaped micrograph by John Koivula. twinned areas (figure 12).Even in very small areas, however, the acute included angle of the twinning in the synthetics can be distinguished from the 60Â or 120Â°anglecharacteristic of Brazil law twinning in natural amethyst. Citrine that has been produced by heating nat- ural amethyst will behave in the polariscope in the same manner as natural amethyst. Bi-colored quartz, sometimes called "ametrine" or ame- thyst/citrine (figure 131, usually behaves differ- ently. Koivula (1980)noted that Brazil twinning is present in the amethyst zones, whereas the citrine zones are untwinned (figure 14). Thus, ame-
Identification of Amethyst GEMS & GEMOLOGY Fall 1986 135 Figure 9. These untwinned synthetic amethysts Figure 10. Even a small area of twinning, as provide a striking contrast to their twinnednatu- shown here in rock crystal quartz, indicates nat- ral counterparts when viewed with the polari- ural origin. Photo by David Hargctt. scope. Note the broad color bands that are char- acteristic of untwinned single-crystal material. Photo by David Hargett. sence of twinning before calling a stone "syn- thetic." The presence of recognizable inclusions, thyst/citrine and citrine with twinning present such as those described by Giibelin and Koivula can be identified as natural by this method. (19861, proves natural origin. At one time, "finger- print" inclusions were regarded as proof of natural OTHER EVIDENCE OF NATURAL VS. origin. This is no longer the case since some syn- SYNTHETIC AMETHYST thetic crystals may also have liquid-filled "finger- Since some natural amethyst may be untwinned, print" inclusions (figure 15), especially near the one should consider evidence other than the ab- surface of the crystal. The presence of spicules, or
Figure 11. In the synthetic amethyst manufac- Figure 12. Color zoning coincides with the tured in Japan,a form of twinning appears as flame-like or irregular-shaped twinned area in small irregular-shaped, arrowhead-shaped, or some synthetic amethysts of Japanese manufac- flame-like areas that are very different from the ~ute,as this photo of the stone in figure 11, taken twinning seen in natural amethyst. Magnified without the polarizers, indicates. Magnified 6 X; 6 x; photomicrograph byJohn Koivula. photomicrograph byJohn Koivula.
136 Identification of Amethyst GEMS & GEMOLOGY Fall 1986 Figure 13. This bi-colored-quartzis also known Figure 14. When amethyst/citrine quartz is os "ametrine" or amethyst/citrine. Photo by viewed with a polariscope, the presence of David Hargett. twinning in the amethyst section indicates that the stone is natural. Photo by Shane McClure.
"nailhead," inclusions (observed very rarely) proves hydrothermal synthetic origin. Occasion- The pigmenting of amethyst is associated with ally, portions of the seed crystal will occur in a Brazil twinning, and thus characteristically the synthetic stone and they are highly diagnostic. deepest color lies~nderthe major rhombohedron, They usually appear as a slightly cloudy white or whereas the sections under the minor rhombo- yellowish iglane often peppered with high-relief "breadcrumbs" (figure 16). Although "bread- Figure 16. "Breadcrumb" inclusions, although crumbs" may (very rarely) be found in natural seen (very rarely) in natural amethyst, usually amethyst, a gemologist seeing them in an other- provide a good clue that the host stone is syn- wise flawless, untwinned stone would be well ad- thetic, as shown here in a synthetic amethyst of vised not to call it natural! Japanese manufacture. Magnified 45 X; In the course of this study, 105 natural ame- photomicrograph by David Hargett. thysts were examined with regard to color zoning.
Figure 15. Liquid-filled fingerprint inclusions are now sometimes seen in synthetic amethyst, as shown here, as well as in natural stones. Magni- fied 20 X; photomicrograph by David Hargett.
a ft, /Â
Identification of Amethyst GEMS & GEMOLOGY Fall 1986 137 Figure 18. When color zoningis seen in a syn- thetic amethyst (here, 10.86 ct), it is limited to Figure 17. This basal section of amethyst clearly darker and lighter shades of purple. Photo by illustrates the deeply pigmented sectors under Shane McClure. the major rhombohedron and the nearly color- less sectors under the minor rhombohedron that are characteristic of color zoning in natural ame- thyst. Photo by Cornelius Hurlbut. hedron may be colorless or nearly so (figure 17). Even under the major rhombohedron the color may not be uniform. According to Frondel (1962), "The amethyst color may be equally developed in successive twin lamallae, but more often alternate lamellae, either right or left, are selectively pig- mented." Color zoning was present in all 105 natural amethysts examined. In a few, zoning was in one Figure 19. The color zoning in natural amethyst direction only, that is, parallel to but one rhombo- (here, 7.04 ct) is typically straight or angular in hedron face. In most, color zoning was parallel to two or three directions, and consists of zones of two or three of the rhombohedron faces from purple and violetish blue or colorless material. which the stones were cut. Brazil twinning was Photo by Shane McClure. present in all but two of the stones. One of these showed color banding parallel to two rhombo- direction of color zoning. But if the zoning is in two hedron faces; the other had color banding parallel or three directions, a natural origin is indicated. It to three. should be noted that the zoning observed in most In the limited number (6)of faceted synthetic synthetic stones is limited to darker and lighter amethysts examined for color zoning, all were un- shades of purple (figure 18);parallel zones of purple twinned. Five were of uniform color throughout, and violetish blue or colorless material and angu- which is rarely if ever seen in natural amethyst lar zoning indicate natural amethyst (see figure (again, see figure 17). One showed imperfect color 191. Other gemologists (e.g., Sondra Francis, pers. zoning parallel to a rhombohedron face that was comm., who has studied this phenomenon in interpreted as being parallel to the seed from depth) report that many synthetic amethysts are which the crystal was grown. In addition to the cut indeed zoned but confirm the zoning differences stones, there was available a piece of synthetic noted above between natural and synthetic stones. rough which included the colorless seed plate cut In recent years, gemological and mineralogical parallel to a rhombohedron face. This was flaw- investigators have noted in print the rippled frac- less, untwinned material with a faint color zoning ture surface of natural amethyst and even outlined parallel to the seed. The observations indicate an a procedure for frosting the surface of an unknown uncertain identity of an untwinned stone with one amethyst (by slight regrinding with fine grit on a
138 Identification of Amethyst GEMS & GEMOLOGY Fall 1986 soft wheel at low speed) so that it is possible to see the complete absence of zoning indicates syn- with the unaided eye the twin lamellae of the thetic origin, but does not provide conclusive proof natural stone [Schneider and Droschel, 1983). Of of origin. Characteristic inclusions can also pro- course, the stone must then be repolished. vide proof of origin. Considerable attention has also been paid in If no other conclusive evidence of origin is Europe to attempts to distinguish amethysts by available, infrared spectrometry appears to contain infrared spectrometry (Katz, 1962; Chakraborty such proof, once the practical aspects of routine and Lehmann, 1978; Zecchini, 1976; and Lind and application have been resolved. Ongoing research Schmetzer, 1983).Problems with specimen orien- in this area promises to solve the few remaining tation, as well as the practical difficulties encoun- difficulties in identifying natural and synthetic tered in transmitting spectral signals through amethyst. However, this study indicates that the faceted gemstones, have thus far thwarted at- overwhelming majority of stones on the market tempts to provide a routine method of identifica- today can be distinguished by the methods already tion through infrared spectra. However, new infra- available. red instrumentation is now available, and results to date are promising for this approach to the sepa- ration of natural and synthetic amethyst. REFERENCES CONCLUSION Balakirev V.G., Tsinober L.I., Tsyganov E.M.(1975) Electron microscope study of Brazil twins in synthetic amethysts. While some "experienced" amethyst dealers have Soviet Physical Crystallography, Vol. 19, pp. 517-520. reported no difficulty in separating synthetic ame- Balitsky V.S. (1980)Synthetic amethyst: Its history, methods of growing, morphology, and peculiar features. Zeitscl~riftder thysts from natural stones merely by looking, Deutschen Gemnibfogischen Gesellscl~uft,Vol. 29, pp. others who ,have mastered the "Schmetzer" test 5- 16. have reported that their stock is badly mixed. One Chakraborty P., Lehn~annG. (1978)Distribution of OH in syn- thetic and natural quartz crystals. Journal of Solid State- dealer stated that even parcels from his own cut- Chemistry, Vol. 17, pp. 305-31 1. ting shops abroad have had as much as 25% syn- Frondel C. (1962)Dana's System of Mineralogy, 7th eel., Vol. 111: thetic amethyst mixed with the natural. It is the Silica Minerals, John Wiley & Sons, New York. Giibelin E. J., Koivula J. I. (19861 Photoatlas of Inclusions in authors' hope that this article will serve to reas- Gemstones. ABC Edition, Zurich. sure the ethical trade that this no longer need be Katz A. (1962)Hydrogen in quartz. Phillips Research Reports, the case, that the practice of salting synthetics into Vol. 17, pp. 133- 135 and 201 -279. parcels of natural stones can be effectively Koivula J.I. (1980) Citrine-amethyst quartz-a gemologically new material. Gems ei) Gemology, Vol. 16, No. 9, pp. deterred. 290-293. Although the reader is reminded that the use of Lind Th., Schmetzer K. (1983) A method for measuring the infrared spectra of faceted gems such as natural and syn- twinning cannot be 100% effective, in preparing thetic amethysts.fourna1 of Gemmology, Vol. 18, No. 5, pp. for this article the authors tested more than 1300 4 11-420. amethysts of known origin [over 1100 natural and Lind Th., Schmetzer K., Bank H. (1983)Untersuchungsmetho- den zur Unterscheidung von natiirlichen und synthetischen 200 synthetic).By noting inclusions, color zoning, Amethysten. Zeitschrift der Deiitschen Gemmologischen and twinning, all the stones could be satisfactorily Gesallschufr, Vol. 32, No. 2/3, pp. 126- 137. identified. Schltjssin H.H., Lang A.R.  A study of repeated twinning, lattice imperfections and impurity distribution in ame- On the basis of information currently avail- thyst. Philosophical Magazine, Vol. 12, pp. 283-296, able, we conclude that the presence of Brazil law Schmetzer K. [I9851 Ein verbesserter Probenhalter und seine twinning in an otherwise flawless amethyst Anwendung auf Problen~eder Unterscheidung natiiralicher und synthetischer Rubine sowie naturlicher und synth- proves natural origin. However, twinning in the etischer Amethyste. Zeitschrift der Deiitschen Gem- form of an acutely angled, flame-like pattern posi- mologischen Gesellschaft, Vol. 34, pp. 30-47. tively identifies synthetic origin. With practice, Schrnetzer K. (1986)An improved sample holder and its use in the distinction of natural and synthetic ruby as well as there should be no confusion between these natural and synthetic amethyst. Journal of Gemmology, twinning patterns. Vol. 20, No. 1, pp. 20-33, In the absence of twinning, synthetic origin is Schneider W.L., Droschel R. (19831Beobachtungen an polysyn- thetisch verzwillingten Quarzen-ein Beitrag zur Unter- probable, but further evidence is required to make scheidung naturlicher und synthetischer Amethyste. a positive identification. Angular or straight zon- Zeitschrift der Deufschen Gemmologischen Gesellscl~uft, ing with colorless or violetish blue zones next to Vol. 32, pp. 28-38. Zecchini P. (1976) fitude de Itabsorption infrarouge de quartz purple areas characterizes natural amethyst. The d'origine naturelle ou de synthese. Revue de Gemmologie presence of zones of only light and dark purple or Association Francaise de Gemmologie, No. 60, pp. 14- 17.
Identification of Amethyst GEMS & GEMOLOGY Fall 1986 139