he quality and value of faceted gem diamonds are often described in terms of the "four C's": carat weight, color, clarity, and cut. Weight is the most objective, because it is measured directly on a balance. Color and clarity are factors for which grading standards have been established by GIA, among others. Cut, however, is much less tractable. Clamor for the standardization of cut, and calls for a simple cut grading system, have been heard sporadically over the last 25 years, gaining strength recently (Shor, 1993(Shor, , 1997Nestlebaum, 1996Nestlebaum, , 1997. Unlike color and clarity, for which diamond trading, consistent teaching, and laboratory practice have created a general consensus, there are a number of different systems for grading cut in round brilliants. As discussed in greater detail later in this article, these systems are based on relatively simple assumptions about the relationship between the proportions and appearance of the round brilliant diamond. Inherent in these systems is the premise that there is one set (or a narrow range) of preferred proportions for round brilliants, and that any deviation from this set of proportions diminishes the attractiveness of a diamond. In this article, we present and discuss our findings with regard to the rather complex relationship between cut proportions and brilliance.Diamond manufacturing has undergone considerable change during this century. For the most part, diamonds are cut within very close proportion tolerances, both to save weight while maximizing appearance and to account for local market preferences (Caspi, 1997). As shown in figure 1 and table 1, however, differences in proportions can produce noticeable differences in appearance in round-brilliant-cut diamonds. Within this single cutting style, there is substantial debate-and some strongly held views-about which proportions yield the best face-up appearance (Federman, 1997 Please see acknowledgments at the end of the article. & Gemology, Vol. 34, No. 3, pp. 158 -183 © 1998 159 material, and on the way these properties govern the paths of light through the faceted gemstone. Gems(Also important are properties particular to each stone, such as polish quality, symmetry, and the presence of inclusions.) Diamond appearance is described chiefly in terms of brilliance (white light returned through the crown), fire (the visible extent of light dispersion into spectral colors), and scintillation (flashes of light reflected from the crown). Yet each of these terms represents a complex appearance concept that has not been defined rigorously, and that cannot be expressed mathematically without making some assumptions and qualifications (see below).Despite the widespread perception in the trade that diamond appearance has been extensively addressed, there is limited information in the literature, and some aspects have never been examined. Several analyses of the round brilliant cut have been published, starting with Wade (1916). Best known are Tolkowsky's (1919) calculations of the proportions that he b...

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Tairus, in Novosibirsk, has produced yet another new type ofRussian hydrothermal synthetic emerald, now being marketed in Bangkok. Examination of eight fac eted samples revealed that, with the exception of certain characteristic inclusions, the basic gemological properties shown by this new synthetic are essentially the same as those enco untered in other hydrothermally grown synthetic emeralds and some natural emeralds. If the characteristic inclusions are not present, distinctive spectral characteristics in both the mid-and near-infrared regions ofthe spectrum will serve to separate these synthetic emeralds from their natural counterparts. The first commercially successful hydrothermal synthesis of beryl is generally attributed to Johann Lechleitner wh o, in 1960, produced hydroth erm al ABOUT THE AUTHORS Mr. Koivula is chief research gemologist, the late Mr.

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