A Simple Procedure to Separate Natural from Synthetic Amethyst on the Basis of Twinning
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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.