diamond, inclusions, gems

Reviews inclusions in diamond.

gems, gemology,

Chudoba and Gübelin provide a compact practical reference to gemology intended both for identifying gemstones and for standardizing the often confusing terminology used in the German gem trade. The principal gemstones and ornamental materials are arranged alphabetically and characterized by their chemical, crystallographic, and physical properties, important localities, trade names, and diagnostic features. Separate sections treat inclusions in natural gemstones, synthetic stones, doublets and triplets; tables of gemological constants; coated stones; synthetic gems and other imitations; the gemological microscope; and practical identification tables. Particular emphasis is placed on simple tests that can be performed with readily available gemological instruments and on microscopic inclusions as a means of distinguishing natural stones from synthetics and assembled or imitation materials. Conceived as a concise working handbook rather than a comprehensive mineralogical treatise, it became the basis for the substantially enlarged second edition of 1966, retitled Edelsteinkundliches Handbuch, and a further expanded third edition in 1974.

diamond, formation, geology, gems

Sinkankas: A truly monumental work of increasing scientific importance, not so much for its information on diamond and its mining as for its detailed studies of the petrology and mineralogy of kimberlitic pipe-fillings. Such treatment of originally deep-seated mantle-derived rocks was well ahead of its time in providing clues as to the nature of the mantle and the formation of the crust in general. The significance of Williams’ studies escaped even himself, and certainly most others, as evidenced in the niggardly review accorded the work by E.H. Kraus, American Mineralogist, 18, 4, 1933, 176, who saw it as merely “supplementing and bringing up-to-date the monumental and classic work of his father.” Because of the greatly increased interest in mantle problems and plate tectonics, the work now appears to be one of those inadvertently fortunate amassments of accurate data which find applications at a later time. The lukewarm reception accorded the book resulted in the issue of relatively few copies, and the book is now difficult to find. It is far from a “popular” presentation, such as might be said about his father’s general work on the diamond mines of South Africa, despite the remark of Kraus noted above, and is most useful to the petrologist and geologist rather than to the gemologist, although it does contain a great deal of interest to the latter as well. Volume I opens with a short chapter on modern mining practice at Kimberley and then describes the geological relationships of the intrusive kimberlites to host rocks and possible origins, with details on the kimberlites themselves. Practically all of this volume is devoted to petrography. The second volume continues similar studies with emphasis upon the inclusions in kimberlites, but now with much space devoted to the diamond itself, taking up such matters as the characteristics of diamonds from various mines, speculations as to their genesis, inclusions in diamond, crystallography and X-ray analyses, physical properties, chemical composition, and growth phenomena. The final chapter treats diamondiferous alluvial deposits of South Africa and the diamonds found therein. There is a long table of diamonds that weighed over 100 carats on pp. 619–636; it is mostly a listing of crystals from the Kimberley district, although some large Brazilian stones are included in an appended list. As may be expected from a largely petrological work, many of the color plates depict kimberlite thin sections with careful descriptions of the minerals therein, but other plates are of more general interest, one showing the now-famous collection of colored diamond crystals formed by the author during his tenure in De Beers. From the gemologist’s standpoint, the work is deemed most important for its studies on inclusions in diamond. Extended review by A.T.J. Dollar, The Gemmologist, 2, 16, 1932, 111–115; 18, 1933, 181–183; 19, 205–207; 21, 268–272; 22, 305–310.

corundum, ruby, sapphire, Sri Lanka.12, gems

A concise account of the crystalline rocks of Ceylon, intended to summarize their character and classification as then understood. Coomaraswamy discusses the terminology of “gneiss” and “granulite,” preferring the latter for many Ceylonese rocks, and regards them as belonging to the Charnockite Series described from southern India, the two regions forming a closely related petrographical province. He describes the principal rock types and their mineral banding, together with the crystalline limestones and rocks intermediate between limestone and granulite. The latter may contain diopside, phlogopite, amphibole, scapolite, spinel, sphene, calcite and, more rarely, serendibite, and may occur at limestone–granulite contacts, as inclusions, dykes or bands; some also yield mica of commercial value. Coomaraswamy argues that, whatever their ultimate origin, the crystalline limestones were essentially contemporaneous with the granulites and crystallized under broadly comparable high-temperature conditions. He also distinguishes the wollastonite-bearing Galle group and discusses the structural geology of the crystalline series, particularly foliation, folding and the influence of rock structure on the development of parallel strike valleys such as those around Hatton. The paper is an important early synthesis of Ceylon’s Precambrian crystalline geology and provides geological context for the mineral and gem deposits associated with these rocks.

gems, gemology, lapidary

Sinkankas: An extremely rare work on gemstones that emphasizes practical aspects of lapidary treatment and other matters that would be of value to the tradesman and jewelry maker. It is further remarkable for including a chapter on the methods by which Indian lacquerware is manufactured, which of course has nothing to do with gems. Gemma discusses the origin of gemstones in the first chapter, considering at length Robert Boyle’s statements on the same as given in his Exercitatio de Origine et Viribus Gemmarum, supporting some and disputing others. Other citations and quotations from authorities appear throughout the text. Other topics treated include the magical and curative powers of gemstones, physical properties, color and its enhancement by use of foils, the pricing of gems, the manufacture of doublets, artificially colored and imitation gems, and substitutes. One chapter is devoted to “figures” in stones, that is, clouds or swarms of inclusions, banding, and other markings which resemble human or other figures, shells in fossiliferous limestones, insects in amber, and stars in star stones. All of these are treated soberly as natural manifestations and are not thought to be magical or miraculous in nature as had been done not too long before Gemma’s time. Another chapter treats petrification due to mineralized waters. From the practical viewpoint, this work is especially rich in details on the cutting of gemstones and the equipment required to do so, including the art of gem engraving. Gem-cutting equipment then in use is described in minute detail, as well as a process for classifying raw tripoli to make it useful as a polishing agent. In all respects this admirable, reasoned work deserves greater recognition.